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/ListToolsResultTests.cs

@@ -0,0 +1,50 @@
+using ModelContextProtocol.Protocol;
+using System.Text.Json;
+using System.Text.Json.Nodes;
+
+namespace ModelContextProtocol.Tests.Protocol;
+
+public static class ListToolsResultTests
+{
+    [Fact]
+    public static void ListToolsResult_SerializationRoundTrip_PreservesAllProperties()
+    {
+        var original = new ListToolsResult
+        {
+            Tools =
+            [
+                new Tool { Name = "get_weather", Description = "Gets weather" },
+                new Tool { Name = "search", Title = "Search Tool" }
+            ],
+            NextCursor = "next-token",
+            Meta = new JsonObject { ["key"] = "value" }
+        };
+
+        string json = JsonSerializer.Serialize(original, McpJsonUtilities.DefaultOptions);
+        var deserialized = JsonSerializer.Deserialize<ListToolsResult>(json, McpJsonUtilities.DefaultOptions);
+
+        Assert.NotNull(deserialized);
+        Assert.Equal(2, deserialized.Tools.Count);
+        Assert.Equal("get_weather", deserialized.Tools[0].Name);
+        Assert.Equal("Gets weather", deserialized.Tools[0].Description);
+        Assert.Equal("search", deserialized.Tools[1].Name);
+        Assert.Equal("Search Tool", deserialized.Tools[1].Title);
+        Assert.Equal("next-token", deserialized.NextCursor);
+        Assert.NotNull(deserialized.Meta);
+        Assert.Equal("value", (string)deserialized.Meta["key"]!);
+    }
+
+    [Fact]
+    public static void ListToolsResult_SerializationRoundTrip_WithMinimalProperties()
+    {
+        var original = new ListToolsResult();
+
+        string json = JsonSerializer.Serialize(original, McpJsonUtilities.DefaultOptions);
+        var deserialized = JsonSerializer.Deserialize<ListToolsResult>(json, McpJsonUtilities.DefaultOptions);
+
+        Assert.NotNull(deserialized);
+        Assert.Empty(deserialized.Tools);
+        Assert.Null(deserialized.NextCursor);
+        Assert.Null(deserialized.Meta);
+    }
+}