Add DebuggerDisplay attributes to Protocol types (#1068)

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: stephentoub <2642209+stephentoub@users.noreply.github.com>
Co-authored-by: Stephen Halter <halter73@gmail.com>

src/ModelContextProtocol.Core/Protocol/PromptMessage.cs

@@ -1,3 +1,4 @@
+using System.Diagnostics;
 using Microsoft.Extensions.AI;
 using System.Text.Json.Serialization;
 
@@ -26,6 +27,7 @@ namespace ModelContextProtocol.Protocol;
 /// See the <see href="https://github.com/modelcontextprotocol/specification/blob/main/schema/">schema</see> for details.
 /// </para>
 /// </remarks>
+[DebuggerDisplay("{DebuggerDisplay,nq}")]
 public sealed class PromptMessage
 {
     /// <summary>
@@ -49,4 +51,19 @@ public sealed class PromptMessage
     /// </remarks>
     [JsonPropertyName("role")]
     public required Role Role { get; set; }
+
+    [DebuggerBrowsable(DebuggerBrowsableState.Never)]
+    private string DebuggerDisplay
+    {
+        get
+        {
+            // Show actual text content if it's a TextContentBlock
+            if (Content is TextContentBlock textBlock)
+            {
+                return $"Role = {Role}, Text = \"{textBlock.Text}\"";
+            }
+
+            return $"Role = {Role}, ContentType = {Content.Type}";
+        }
+    }
 }