Add serialization roundtrip tests for all Protocol namespace types (#1289)

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

tests/ModelContextProtocol.Tests/Protocol/CreateMessageResultTests.cs

@@ -164,6 +164,7 @@ public class CreateMessageResultTests
         Assert.Equal(2, deserialized.Content.Count);
         Assert.Equal("toolUse", deserialized.StopReason);
         Assert.NotNull(deserialized.Meta);
+        Assert.Equal("metadata", (string)deserialized.Meta["custom"]!);
     }
 
     [Fact]