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/CancelMcpTaskResultTests.cs

@@ -0,0 +1,33 @@
+using ModelContextProtocol.Protocol;
+using System.Text.Json;
+
+namespace ModelContextProtocol.Tests.Protocol;
+
+public static class CancelMcpTaskResultTests
+{
+    [Fact]
+    public static void CancelMcpTaskResult_SerializationRoundTrip()
+    {
+        // Arrange
+        var original = new CancelMcpTaskResult
+        {
+            TaskId = "cancelled-789",
+            Status = McpTaskStatus.Cancelled,
+            StatusMessage = "Cancelled by user",
+            CreatedAt = DateTimeOffset.UtcNow,
+            LastUpdatedAt = DateTimeOffset.UtcNow,
+            TimeToLive = null,
+            PollInterval = null
+        };
+
+        // Act
+        string json = JsonSerializer.Serialize(original, McpJsonUtilities.DefaultOptions);
+        var deserialized = JsonSerializer.Deserialize<CancelMcpTaskResult>(json, McpJsonUtilities.DefaultOptions);
+
+        // Assert
+        Assert.NotNull(deserialized);
+        Assert.Equal(original.TaskId, deserialized.TaskId);
+        Assert.Equal(original.Status, deserialized.Status);
+        Assert.Equal(original.StatusMessage, deserialized.StatusMessage);
+    }
+}