Add real WebSocket integration tests to dart_node_ws

[ajayi-joseph]

Description

This PR implements real integration tests for the dart_node_ws package, replacing the previous TODO. The new tests verify actual client-server WebSocket communication using Node.js ws for the client via JS interop. All tests pass, and the implementation follows project guidelines (typed APIs, no duplication, Result<T,E> usage, and no global state).

• Adds 3 new integration tests to ws_test.dart (total: 14 tests)
• Removes obsolete websocket_test.dart (if present)
• Ensures only ws_test.dart is used for WebSocket tests
• All tests pass locally

Checklist

• Integration tests implemented and passing
• No duplicate or obsolete test files
• Follows project code and testing rules
• Clean commit history