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/ListResourceTemplatesResultTests.cs

@@ -0,0 +1,55 @@
+using ModelContextProtocol.Protocol;
+using System.Text.Json;
+using System.Text.Json.Nodes;
+
+namespace ModelContextProtocol.Tests.Protocol;
+
+public static class ListResourceTemplatesResultTests
+{
+    [Fact]
+    public static void ListResourceTemplatesResult_SerializationRoundTrip_PreservesAllProperties()
+    {
+        var original = new ListResourceTemplatesResult
+        {
+            ResourceTemplates =
+            [
+                new ResourceTemplate
+                {
+                    Name = "document",
+                    UriTemplate = "file:///{path}",
+                    Description = "A document",
+                    MimeType = "text/plain"
+                }
+            ],
+            NextCursor = "cursor-abc",
+            Meta = new JsonObject { ["key"] = "value" }
+        };
+
+        string json = JsonSerializer.Serialize(original, McpJsonUtilities.DefaultOptions);
+        var deserialized = JsonSerializer.Deserialize<ListResourceTemplatesResult>(json, McpJsonUtilities.DefaultOptions);
+
+        Assert.NotNull(deserialized);
+        Assert.Single(deserialized.ResourceTemplates);
+        Assert.Equal("document", deserialized.ResourceTemplates[0].Name);
+        Assert.Equal("file:///{path}", deserialized.ResourceTemplates[0].UriTemplate);
+        Assert.Equal("A document", deserialized.ResourceTemplates[0].Description);
+        Assert.Equal("text/plain", deserialized.ResourceTemplates[0].MimeType);
+        Assert.Equal("cursor-abc", deserialized.NextCursor);
+        Assert.NotNull(deserialized.Meta);
+        Assert.Equal("value", (string)deserialized.Meta["key"]!);
+    }
+
+    [Fact]
+    public static void ListResourceTemplatesResult_SerializationRoundTrip_WithMinimalProperties()
+    {
+        var original = new ListResourceTemplatesResult();
+
+        string json = JsonSerializer.Serialize(original, McpJsonUtilities.DefaultOptions);
+        var deserialized = JsonSerializer.Deserialize<ListResourceTemplatesResult>(json, McpJsonUtilities.DefaultOptions);
+
+        Assert.NotNull(deserialized);
+        Assert.Empty(deserialized.ResourceTemplates);
+        Assert.Null(deserialized.NextCursor);
+        Assert.Null(deserialized.Meta);
+    }
+}