MessageSegments.json

{ "body" : 
   {
      "messageSegments" : [ 
         {
            "type": "mention",  // <== type = mention for @mentions
            "id":"005D0000001LL8YIAW" <== user id for the @mention 
          },
          {
            "type": "link",
            "href":"http://www.google.com"
          },
          {
             "type": "text",
             "text":" Did you see the new plans?"} ]
           }
      ]
   }
}

All that is wrong.
You want to create an attachment with this body json:

{ "attachment": 
   {
      "attachmentType":"Link",
      "url": "http://www.salesforce.com",
      "urlName": "Salesforce"
   }
}