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/ArgumentTests.cs

@@ -0,0 +1,24 @@
+using ModelContextProtocol.Protocol;
+using System.Text.Json;
+
+namespace ModelContextProtocol.Tests.Protocol;
+
+public static class ArgumentTests
+{
+    [Fact]
+    public static void Argument_SerializationRoundTrip_PreservesAllProperties()
+    {
+        var original = new Argument
+        {
+            Name = "temperature",
+            Value = "72"
+        };
+
+        string json = JsonSerializer.Serialize(original, McpJsonUtilities.DefaultOptions);
+        var deserialized = JsonSerializer.Deserialize<Argument>(json, McpJsonUtilities.DefaultOptions);
+
+        Assert.NotNull(deserialized);
+        Assert.Equal(original.Name, deserialized.Name);
+        Assert.Equal(original.Value, deserialized.Value);
+    }
+}