Use ReadOnlyMemory<byte> for binary data to eliminate UTF-16 transcoding (#1070)

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: ericstj <8918108+ericstj@users.noreply.github.com>
Co-authored-by: Eric StJohn <ericstj@microsoft.com>

src/ModelContextProtocol.Core/Protocol/DebuggerDisplayHelper.cs

@@ -27,4 +27,25 @@ internal static class DebuggerDisplayHelper
 
         return "invalid base64";
     }
+
+    /// <summary>
+    /// Gets the decoded length of base64 data encoded as UTF-8 bytes for debugger display.
+    /// </summary>
+    internal static string GetBase64LengthDisplay(ReadOnlyMemory<byte> base64Data)
+    {
+#if NET
+        if (System.Buffers.Text.Base64.IsValid(base64Data.Span, out int decodedLength))
+        {
+            return $"{decodedLength} bytes";
+        }
+#else
+        try
+        {
+            return $"{Convert.FromBase64String(System.Text.Encoding.UTF8.GetString(base64Data.ToArray())).Length} bytes";
+        }
+        catch { }
+#endif
+
+        return "invalid base64";
+    }
 }