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/CreateTaskResultTests.cs

@@ -0,0 +1,41 @@
+using ModelContextProtocol.Protocol;
+using System.Text.Json;
+using System.Text.Json.Nodes;
+
+namespace ModelContextProtocol.Tests.Protocol;
+
+public static class CreateTaskResultTests
+{
+    [Fact]
+    public static void CreateTaskResult_SerializationRoundTrip_PreservesAllProperties()
+    {
+        var original = new CreateTaskResult
+        {
+            Task = new McpTask
+            {
+                TaskId = "task-123",
+                Status = McpTaskStatus.Working,
+                StatusMessage = "Processing",
+                CreatedAt = new DateTimeOffset(2025, 6, 1, 12, 0, 0, TimeSpan.Zero),
+                LastUpdatedAt = new DateTimeOffset(2025, 6, 1, 12, 5, 0, TimeSpan.Zero),
+                TimeToLive = TimeSpan.FromHours(1),
+                PollInterval = TimeSpan.FromSeconds(5)
+            },
+            Meta = new JsonObject { ["key"] = "value" }
+        };
+
+        string json = JsonSerializer.Serialize(original, McpJsonUtilities.DefaultOptions);
+        var deserialized = JsonSerializer.Deserialize<CreateTaskResult>(json, McpJsonUtilities.DefaultOptions);
+
+        Assert.NotNull(deserialized);
+        Assert.Equal("task-123", deserialized.Task.TaskId);
+        Assert.Equal(McpTaskStatus.Working, deserialized.Task.Status);
+        Assert.Equal("Processing", deserialized.Task.StatusMessage);
+        Assert.Equal(original.Task.CreatedAt, deserialized.Task.CreatedAt);
+        Assert.Equal(original.Task.LastUpdatedAt, deserialized.Task.LastUpdatedAt);
+        Assert.Equal(original.Task.TimeToLive, deserialized.Task.TimeToLive);
+        Assert.Equal(original.Task.PollInterval, deserialized.Task.PollInterval);
+        Assert.NotNull(deserialized.Meta);
+        Assert.Equal("value", (string)deserialized.Meta["key"]!);
+    }
+}