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/JsonRpcErrorTests.cs

@@ -0,0 +1,55 @@
+using ModelContextProtocol.Protocol;
+using System.Text.Json;
+
+namespace ModelContextProtocol.Tests.Protocol;
+
+public static class JsonRpcErrorTests
+{
+    [Fact]
+    public static void JsonRpcError_SerializationRoundTrip_PreservesAllProperties()
+    {
+        var original = new JsonRpcError
+        {
+            Id = new RequestId(1),
+            Error = new JsonRpcErrorDetail
+            {
+                Code = -32600,
+                Message = "Invalid Request",
+                Data = "Additional error context"
+            }
+        };
+
+        string json = JsonSerializer.Serialize(original, McpJsonUtilities.DefaultOptions);
+        var deserialized = JsonSerializer.Deserialize<JsonRpcMessage>(json, McpJsonUtilities.DefaultOptions);
+
+        Assert.NotNull(deserialized);
+        var error = Assert.IsType<JsonRpcError>(deserialized);
+        Assert.Equal(original.Id, error.Id);
+        Assert.Equal(-32600, error.Error.Code);
+        Assert.Equal("Invalid Request", error.Error.Message);
+        Assert.NotNull(error.Error.Data);
+    }
+
+    [Fact]
+    public static void JsonRpcError_SerializationRoundTrip_WithoutOptionalData()
+    {
+        var original = new JsonRpcError
+        {
+            Id = new RequestId("err-42"),
+            Error = new JsonRpcErrorDetail
+            {
+                Code = -32601,
+                Message = "Method not found"
+            }
+        };
+
+        string json = JsonSerializer.Serialize(original, McpJsonUtilities.DefaultOptions);
+        var deserialized = JsonSerializer.Deserialize<JsonRpcMessage>(json, McpJsonUtilities.DefaultOptions);
+
+        Assert.NotNull(deserialized);
+        var error = Assert.IsType<JsonRpcError>(deserialized);
+        Assert.Equal(original.Id, error.Id);
+        Assert.Equal(-32601, error.Error.Code);
+        Assert.Equal("Method not found", error.Error.Message);
+    }
+}