Adding/Throwing validation errors from within domain objects/services instead of using Results Pattern

Program.cs

using FastEndpoints;
using FastEndpoints.Swagger;

var bld = WebApplication.CreateBuilder();
bld.Services
   .AddFastEndpoints()
   .SwaggerDocument();

var app = bld.Build();
app.UseFastEndpoints()
   .UseSwaggerGen();
app.Run();

sealed class Request
{
    public bool IsHappyPath { get; set; }
}

sealed class Response
{
    public string Message { get; set; }
}

sealed class TestEndpoint : Endpoint<Request, Response>
{
    public override void Configure()
    {
        Get("test/{IsHappyPath}");
        AllowAnonymous();
    }

    public override async Task HandleAsync(Request r, CancellationToken c)
    {
        var msg = HelloService.SayHello(r.IsHappyPath);

        //if the hello service throws an error via ValidationContext,
        //execution doesn't go past this point and an automatic 400 response is sent with the details.

        //if the hello service simply adds errors instead of throwing,
        //you can access the errors via this.ValidationFailures property:

        await SendAsync(new()
        {
            Message = msg
        });
    }
}

sealed class HelloService
{
    public static string SayHello(bool isHappyPath)
    {
        if (!isHappyPath)
        {
            //https://fast-endpoints.com/docs/validation#throwing-adding-errors-from-anywhere
            var valCtx = ValidationContext.Instance;
            valCtx.ThrowError("I am unhappy!");
        }
        return "Hello world!";
    }
}