docs: add missing xml comments

src/AnthropicClient/Models/MessageStopEventData.cs

@@ -1,7 +1,14 @@
 namespace AnthropicClient.Models;
 
+/// <summary>
+/// Represents data for a message_stop event.
+/// </summary>
 public class MessageStopEventData : EventData
 {
+  /// <summary>
+  /// Initializes a new instance of the <see cref="MessageStopEventData"/> class.
+  /// </summary>
+  /// <returns>A new instance of the <see cref="MessageStopEventData"/> class.</returns>
   public MessageStopEventData() : base(EventType.MessageStop)
   {
   }