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/ListTasksResultTests.cs

@@ -0,0 +1,46 @@
+using ModelContextProtocol.Protocol;
+using System.Text.Json;
+
+namespace ModelContextProtocol.Tests.Protocol;
+
+public static class ListTasksResultTests
+{
+    [Fact]
+    public static void ListTasksResult_SerializationRoundTrip()
+    {
+        // Arrange
+        var original = new ListTasksResult
+        {
+            Tasks =
+            [
+                new McpTask
+                {
+                    TaskId = "task-1",
+                    Status = McpTaskStatus.Working,
+                    CreatedAt = DateTimeOffset.UtcNow,
+                    LastUpdatedAt = DateTimeOffset.UtcNow
+                },
+                new McpTask
+                {
+                    TaskId = "task-2",
+                    Status = McpTaskStatus.Completed,
+                    CreatedAt = DateTimeOffset.UtcNow,
+                    LastUpdatedAt = DateTimeOffset.UtcNow
+                }
+            ],
+            NextCursor = "next-page-token"
+        };
+
+        // Act
+        string json = JsonSerializer.Serialize(original, McpJsonUtilities.DefaultOptions);
+        var deserialized = JsonSerializer.Deserialize<ListTasksResult>(json, McpJsonUtilities.DefaultOptions);
+
+        // Assert
+        Assert.NotNull(deserialized);
+        Assert.NotNull(deserialized.Tasks);
+        Assert.Equal(2, deserialized.Tasks.Length);
+        Assert.Equal(original.Tasks[0].TaskId, deserialized.Tasks[0].TaskId);
+        Assert.Equal(original.Tasks[1].TaskId, deserialized.Tasks[1].TaskId);
+        Assert.Equal(original.NextCursor, deserialized.NextCursor);
+    }
+}