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/ListResourcesResultTests.cs

@@ -0,0 +1,53 @@
+using ModelContextProtocol.Protocol;
+using System.Text.Json;
+using System.Text.Json.Nodes;
+
+namespace ModelContextProtocol.Tests.Protocol;
+
+public static class ListResourcesResultTests
+{
+    [Fact]
+    public static void ListResourcesResult_SerializationRoundTrip_PreservesAllProperties()
+    {
+        var original = new ListResourcesResult
+        {
+            Resources =
+            [
+                new Resource
+                {
+                    Uri = "file:///readme.md",
+                    Name = "README",
+                    MimeType = "text/markdown"
+                }
+            ],
+            NextCursor = "next-page",
+            Meta = new JsonObject { ["key"] = "value" }
+        };
+
+        string json = JsonSerializer.Serialize(original, McpJsonUtilities.DefaultOptions);
+        var deserialized = JsonSerializer.Deserialize<ListResourcesResult>(json, McpJsonUtilities.DefaultOptions);
+
+        Assert.NotNull(deserialized);
+        Assert.Single(deserialized.Resources);
+        Assert.Equal("file:///readme.md", deserialized.Resources[0].Uri);
+        Assert.Equal("README", deserialized.Resources[0].Name);
+        Assert.Equal("text/markdown", deserialized.Resources[0].MimeType);
+        Assert.Equal("next-page", deserialized.NextCursor);
+        Assert.NotNull(deserialized.Meta);
+        Assert.Equal("value", (string)deserialized.Meta["key"]!);
+    }
+
+    [Fact]
+    public static void ListResourcesResult_SerializationRoundTrip_WithMinimalProperties()
+    {
+        var original = new ListResourcesResult();
+
+        string json = JsonSerializer.Serialize(original, McpJsonUtilities.DefaultOptions);
+        var deserialized = JsonSerializer.Deserialize<ListResourcesResult>(json, McpJsonUtilities.DefaultOptions);
+
+        Assert.NotNull(deserialized);
+        Assert.Empty(deserialized.Resources);
+        Assert.Null(deserialized.NextCursor);
+        Assert.Null(deserialized.Meta);
+    }
+}