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/DebuggerDisplayHelper.cs

@@ -0,0 +1,30 @@
+using System;
+
+namespace ModelContextProtocol.Protocol;
+
+/// <summary>
+/// Internal helper methods for DebuggerDisplay implementations.
+/// </summary>
+internal static class DebuggerDisplayHelper
+{
+    /// <summary>
+    /// Gets the decoded length of base64 data for debugger display.
+    /// </summary>
+    internal static string GetBase64LengthDisplay(string base64Data)
+    {
+#if NET
+        if (System.Buffers.Text.Base64.IsValid(base64Data, out int decodedLength))
+        {
+            return $"{decodedLength} bytes";
+        }
+#else
+        try
+        {
+            return $"{Convert.FromBase64String(base64Data).Length} bytes";
+        }
+        catch { }
+#endif
+
+        return "invalid base64";
+    }
+}