https://docs.microsoft.com/en-us/microsoftteams/platform/concepts/bots/bot-conversations/bots-conversations : Microsoft Teams Outgoing Webhook Message Parser. Aren't there parser exists for this not useful teams message format?

Main.cs

void Main()
{
    var sources = new[]
    {
        "<at>cibot</at><strong>すとろんぐ</strong>",
        "<at>cibot</at><strong>すとろんぐ</strong>ほげもげ",
        "ほげもげ<at>cibot</at><strong>すとろんぐ</strong>ほげもげ",
        "<at>cibot</at> こんにちは\n",
        "<at>cibot</at> <img alt=\"404 エラー - 見つかりません\" src=\"https://as-api.asm.skype.com/v1/objects/0-ea-d5-f9a6e7404b54cf7e36924f8e684d3e36/views/imgo\" id=\"x_0-ea-d5-f9a6e7404b54cf7e36924f8e684d3e36\" itemscope=\"\" itemtype=\"http://schema.skype.com/AMSImage\" style=\"width:375px; height:250px\">\n",
        "<at>cibot</at>&nbsp;\n",
        "<at>cibot</at>&nbsp;こんちには\n",
        "<at>cibot</at> <at><img alt=\"😁\" itemid=\"laugh\" itemscope=\"\" itemtype=\"http://schema.skype.com/Emoji\" src=\"https://statics.teams.microsoft.com/evergreen-assets/funstuff/skype-emoticons-f/laugh/default_20.png\" style=\"width:20px; height:20px\"></at>\n",
        @"<at>cibot</at> hoge\n",
        "<at>cibot</at> <at><img alt=\"❤\" itemid=\"heart\" itemscope=\"\" itemtype=\"http://schema.skype.com/Emoji\" src=\"https://statics.teams.microsoft.com/evergreen-assets/funstuff/skype-emoticons-f/heart/default_20.png\" style=\"width:20px; height:20px\"></at>",
        "<at>cibot</at> <at><img alt=\"❤️\" itemid=\"heart\" itemscope=\"\" itemtype=\"http://schema.skype.com/Emoji\" src=\"https://statics.teams.microsoft.com/evergreen-assets/funstuff/skype-emoticons-f/heart/default_20.png\" style=\"width:20px; height:20px\"></at><at><img alt=\"🤗\" itemid=\"hug\" itemscope=\"\" itemtype=\"http://schema.skype.com/Emoji\" src=\"https://statics.teams.microsoft.com/evergreen-assets/funstuff/skype-emoticons-f/hug/default_20.png\" style=\"width:20px; height:20px\"></at><at><img alt=\"⛄\" itemid=\"snowangel\" itemscope=\"\" itemtype=\"http://schema.skype.com/Emoji\" src=\"https://statics.teams.microsoft.com/evergreen-assets/funstuff/skype-emoticons-f/snowangel/default_20.png\" style=\"width:20px; height:20px\"></at>\n",
        "ほげもげ<at>cibot</at>ほげもげ<strong>すとろんぐ</strong> <at><img alt=\"😁\" itemid=\"laugh\" itemscope=\"\" itemtype=\"http://schema.skype.com/Emoji\" src=\"https://statics.teams.microsoft.com/evergreen-assets/funstuff/skype-emoticons-f/laugh/default_20.png\" style=\"width:20px; height:20px\"></at>という画像らしい\n",
        "<at>cibot</at> <strong>ほげもげ</strong>\n\n<em>ふがうが</em>\n\n<em>ぴよぴよ</em>\n\n<u>ほげまｓｄふぁ</u>\n\n\r\n<blockquote>\n<u>なるほどね</u>\n</blockquote>\r\n\n",
        "<at>cibot</at>&nbsp;<at>ほげもげ</at>\n<strong>ふぃががい</strong>\n\n<em>ぴよぴよ</em>\n\n<u>あああげ</u>\n\n\r\n<blockquote>\n<u>ほあｓｄふぁｄふぁ</u>\n</blockquote>\r\n\n\n<a href=\"https://google.com\" rel=\"noreferrer noopener\" target=\"_blank\" title=\"https://google.com\">ほげもげ</a>\n\n\r\n<h2>asdfadfa</h2>\r\n\n\n",
        "\r\n<div itemprop=\"copy-paste-block\"><at>cibot</at>\n\r\n<div style=\"font-size:14px\"><strong>ほげもげ</strong>\n\n\r\n<div style=\"font-size:14px\"><em>ふがうが</em>\n\n\r\n<div style=\"font-size:14px\"><em>ぴよぴよ</em>\n\n\r\n<div style=\"font-size:14px\"><u>ほげまｓｄふぁ</u>\n\n\r\n<blockquote style=\"font-size:14px\">\n<u>なるほどね</u>\n</blockquote>\r\n\n\n",
    };

    var parser = new TeamsMessageParser();
    foreach (var source in sources)
    {
        parser.Parse(source);
    }

}

TeamsOutgoingWebhookMessageParser.cs

public class TeamsMessageParser
{
    public ParsedMessageDetail Parse(string source)
    {
        var currentIndex = 0;
        var tokens = new List<MessageToken>();

        for (var i = 0; i < source.Length; i++)
        {
            var currentTokenStatus = new MessageToken()
            {
                Index = currentIndex++,
            };

            var begin = 0;
            int end = 0;

            // tag handling
            // <
            if (source[i] == '<' && source[i + 1] != '/')
            {
                // begin tag handling
                (begin, currentTokenStatus.BeginTag, currentTokenStatus.AttributeSource) = DetectBeginTag(source, i);
                i = begin;
                
                // attribute handling
                if (!string.IsNullOrWhiteSpace(currentTokenStatus.AttributeSource))
                {
                    currentTokenStatus.Attributes = ParseAttribute(currentTokenStatus.AttributeSource, 0);
                    currentTokenStatus.AttributeType = currentTokenStatus.Tag;
                }

                // end tag handling
                (end, currentTokenStatus.EndTag, currentTokenStatus.IsTagClosed) = DetectEndTag(source, begin, currentTokenStatus.Tag);

                // content
                var increment = 0;
                if (currentTokenStatus.ContainsAttribute)
                {
                    currentTokenStatus.Content = source.ToCharArray().AsSpan(begin + " ".Length + currentTokenStatus.AttributeSource.Length + ">".Length, end - (begin + " ".Length + currentTokenStatus.AttributeSource.Length)).ToString();
                    increment = currentTokenStatus.Content.Length + " ".Length + currentTokenStatus.AttributeSource.Length + currentTokenStatus.EndTag.Length;
                    
                    // special handling - only img tag should set content manually.
                    if (currentTokenStatus.AttributeType == "img" && string.IsNullOrWhiteSpace(currentTokenStatus.Content))
                    {
                        // 後ろに全角1つ、半角2つの空白ないと文字がかぶったり表示されない... ハック....
                        currentTokenStatus.Content = currentTokenStatus.FullQualifiedBeginTag + " 　  ";
                    }
                }
                else
                {
                    currentTokenStatus.Content = source.ToCharArray().AsSpan(begin + 1, end - i).ToString();
                    increment = currentTokenStatus.Content.Length + currentTokenStatus.EndTag.Length;
                }

                // content contains image attributes
                if (IsImage(currentTokenStatus.Content, 0))
                {
                    var contentBegin = 0;
                    var contentBeginTag = "";
                    var contentAttributeSource = "";
                    
                    // begin tag handling
                    (contentBegin, contentBeginTag, contentAttributeSource) = DetectBeginTag(currentTokenStatus.Content, 0);

                    // attribute handling
                    if (!string.IsNullOrWhiteSpace(contentAttributeSource))
                    {
                        currentTokenStatus.Attributes = ParseAttribute(contentAttributeSource, 0);
                        currentTokenStatus.AttributeType = contentBeginTag.Replace("<", "").Replace(">", "");
                        if (currentTokenStatus.AttributeType == "img")
                        {
                            // 後ろに全角1つ、半角2つの空白ないと文字がかぶったり表示されない... ハック....
                            currentTokenStatus.Content = currentTokenStatus.Content + " 　  ";
                        }
                    }
                }

                // add to result
                tokens.Add(currentTokenStatus);

                // increment index
                i += increment;

                //currentTokenStatus.Dump();
            }
            else
            {
                // content handling
                (begin, currentTokenStatus.Content) = ParseTaglessContent(source, i);
                i += begin;

                // add to result
                tokens.Add(currentTokenStatus);
            }
        }
        
        // Parse against self when Copy-Paste
        if (tokens.Any(x => x.Tag == "div" && x.AttributeType == "div" && x.AttributeSource.Contains("copy-paste-block")))
        {
            var newSource = tokens.Select(x => x.Content).ToJoinedString("");
            return Parse(newSource);
        }

        //tokens.Dump();
        var result = new ParsedMessageDetail()
        {
            Source = source,
            To = tokens.Where(x => x.IsToSection).Select(x => x.Content).ToJoinedString(),
            EchoMessage = tokens.Where(x => !x.IsToSection).Select(x => x.TaggedContent).ToJoinedString(),
            TagglessMessage = tokens.Where(x => !x.IsToSection).Select(x => x.Content).ToJoinedString(),
            Images = tokens.Where(x => !x.IsToSection).Where(x => x.IsImg).Select(x => x.Content).ToJoinedString(),
            Tokens = tokens.ToArray(),
        };
        return result;
    }

    private bool IsImage(string source, int startIndex)
    {
        return source.Length - startIndex > 4 && source.AsSpan(startIndex, 4).ToString() == "<img";
    }

    private (int legnth, string tag, string attribute) DetectBeginTag(string source, int startIndex)
    {
        var tagLength = 0;
        var begintagChars = new List<char>();
        var tagAttributeChars = new List<char>();

        begintagChars.Add(source[startIndex]);
        for (var j = startIndex + 1; j < source.Length; j++)
        {
            // ' '
            if (source[j] == ' ')
            {
                // attribute detection
                tagLength = j;
                for (var k = j + 1; k < source.Length; k++)
                {
                    if (source[k] == '>')
                    {
                        begintagChars.Add(source[k]);
                        break;
                    }
                    tagAttributeChars.Add(source[k]);
                }
                break;
            }

            // '>'
            begintagChars.Add(source[j]);
            if (source[j] == '>')
            {
                tagLength = j;
                break;
            }
        }
        var begintag = begintagChars.ToJoinedString();
        //bgintag.Dump("begintag");
        var attribute = tagAttributeChars.ToJoinedString();
        //attribute.Dump();

        return (tagLength, begintag, attribute);
    }

    private string DetectAttribute(string source, int startIndex)
    {
        var tagAttributeChars = new List<char>();

        for (var j = startIndex + 1; j < source.Length; j++)
        {
            // ' '
            if (source[j] == ' ')
            {
                // attribute detection
                for (var k = j + 1; k < source.Length; k++)
                {
                    if (source[k] == '>')
                    {
                        break;
                    }
                    tagAttributeChars.Add(source[k]);
                }
                break;
            }
        }
        var attribute = tagAttributeChars.ToJoinedString();
        //attribute.Dump();

        return attribute;
    }

    private (int length, string tag, bool closed) DetectEndTag(string source, int startIndex, string tag)
    {
        var length = 0;
        var closed = false;
        string endtag = "";
        var endtagChars = new List<char>();
        for (var j = startIndex + 1; j < source.Length; j++)
        {
            if (source[j] == '<' && source[j + 1] == '/')
            {
                // end tag should match with begin tag
                if (source.AsSpan(j + "</".Length, tag.Length + 1).ToString() == tag + ">")
                {
                    endtagChars.Add(source[j]);
                    for (var k = j + 1; k < source.Length; k++)
                    {
                        endtagChars.Add(source[k]);
                        if (source[k] == '>')
                        {
                            break;
                        }
                    }
                    endtag = string.Join("", endtagChars);
                    closed = true;
                    //endtag.Dump("endtag");
                    break;
                }
            }
            else
            {
                length = j;
            }
        }

        return (length, endtag, closed);
    }

    private (int legnth, string content) ParseTaglessContent(string source, int startIndex)
    {
        var length = 0;
        var contentChars = new List<Char>();
        for (var j = startIndex; j < source.Length; j++)
        {
            length++;
            contentChars.Add(source[j]);

            if (j < source.Length - 1 && source[j + 1] == '<')
            {
                length--;
                break;
            }
        }

        var content = string.Join("", contentChars);
        return (length, content);
    }

    private ContentAttribute[] ParseAttribute(string source, int startIndex)
    {
        var attributes = new List<ContentAttribute>();
        var keyLength = 0;
        for (var j = startIndex; j < source.Length - ">".Length; j++)
        {
            if (source[j] != '=' && source[j + 1] != '"')
            {
                keyLength++;
            }
            else
            {
                var attribute = new ContentAttribute();
                attribute.Key = source.AsSpan(j - keyLength, keyLength).ToString();

                var valueLength = 0;
                for (var k = j + "=\"".Length; k < source.Length; k++)
                {
                    if (source[k] != '"')
                    {
                        valueLength++;
                    }
                    else
                    {
                        attribute.Value = "\"" + source.AsSpan(k - valueLength, valueLength).ToString() + "\"";
                        break;
                    }
                }
                attribute.Source = source.AsSpan(j - attribute.Key.Length, attribute.Key.Length + attribute.Devide.Length + attribute.Value.Length).ToString();
                j += attribute.Source.Length - attribute.Key.Length;
                keyLength = 0;
                valueLength = 0;
                attributes.Add(attribute);
            }
        }
        return attributes.ToArray();
    }
}

public class ParsedMessageDetail
{
    public string Source { get; set; }
    public string To { get; set; }
    public string EchoMessage { get; set; }
    public string TagglessMessage { get; set; }
    public string Images { get; set; }
    public MessageToken[] Tokens { get; set; }
}

public class MessageToken
{
    public int Index { get; set; }
    public string Tag => BeginTag?.Replace("<", "").Replace(">", "");
    public bool IsToSection => Tag == "at" && !IsImg;
    public string BeginTag { get; set; }
    public string FullQualifiedBeginTag => string.IsNullOrWhiteSpace(BeginTag) 
        ? "" 
        : Attributes == null || IsTagClosed
            ? BeginTag.ToCharArray().AsSpan(0, BeginTag.Length - 1).ToString() + BeginTag.ToCharArray().AsSpan(BeginTag.Length - 1, 1).ToString()
            : BeginTag.ToCharArray().AsSpan(0, BeginTag.Length - 1).ToString() + " " + AttributeSource + BeginTag.ToCharArray().AsSpan(BeginTag.Length - 1, 1).ToString();
    public string EndTag { get; set; }
    public bool IsTagClosed { get; set; }
    public string Content { get; set; }
    public bool IsImg => AttributeType == "img" && Attributes != null && Attributes.Any();
    public string TaggedContent => IsImg ? Content : FullQualifiedBeginTag + Content + EndTag;
    public string AttributeSource { get; set; }
    public string AttributeType { get; set; }
    public bool ContainsAttribute => Attributes != null && Attributes.Any();
    public ContentAttribute[] Attributes { get; set; }
}

public class ContentAttribute
{
    public string Source { get; set; }
    public string Devide { get; set; } = "=";
    public string Key { get; set; }
    public string Value { get; set; }
}




public static class EnumerableExtensions
{
    public static string ToJoinedString<T>(this IEnumerable<T> source, string separator = "")
    {
        return string.Join(separator, source);
    }

    public static IEnumerable<TSource> Concat<TSource>(this IEnumerable<TSource> source, params TSource[] values)
    {
        if (source == null) throw new ArgumentNullException("source");

        return source.ConcatCore(values);
    }

    private static IEnumerable<TSource> ConcatCore<TSource>(this IEnumerable<TSource> source, params TSource[] values)
    {
        foreach (var item in source) yield return item;
        foreach (var x in values) yield return x;
    }
}

Result