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/RootTests.cs

@@ -0,0 +1,44 @@
+using ModelContextProtocol.Protocol;
+using System.Text.Json;
+
+namespace ModelContextProtocol.Tests.Protocol;
+
+public static class RootTests
+{
+    [Fact]
+    public static void Root_SerializationRoundTrip_PreservesAllProperties()
+    {
+        var original = new Root
+        {
+            Uri = "file:///home/user/project",
+            Name = "My Project",
+            Meta = JsonDocument.Parse("""{"custom":"data"}""").RootElement.Clone()
+        };
+
+        string json = JsonSerializer.Serialize(original, McpJsonUtilities.DefaultOptions);
+        var deserialized = JsonSerializer.Deserialize<Root>(json, McpJsonUtilities.DefaultOptions);
+
+        Assert.NotNull(deserialized);
+        Assert.Equal("file:///home/user/project", deserialized.Uri);
+        Assert.Equal("My Project", deserialized.Name);
+        Assert.NotNull(deserialized.Meta);
+        Assert.Equal("data", deserialized.Meta.Value.GetProperty("custom").GetString());
+    }
+
+    [Fact]
+    public static void Root_SerializationRoundTrip_WithMinimalProperties()
+    {
+        var original = new Root
+        {
+            Uri = "file:///tmp"
+        };
+
+        string json = JsonSerializer.Serialize(original, McpJsonUtilities.DefaultOptions);
+        var deserialized = JsonSerializer.Deserialize<Root>(json, McpJsonUtilities.DefaultOptions);
+
+        Assert.NotNull(deserialized);
+        Assert.Equal("file:///tmp", deserialized.Uri);
+        Assert.Null(deserialized.Name);
+        Assert.Null(deserialized.Meta);
+    }
+}