test(realtime): add comprehensive integration tests

- Add comprehensive integration tests for Realtime features
- Test connection management, channel management, broadcast, presence, and postgres changes
- Add real application scenario test simulating chat room with 2 clients
- Remove duplicate/redundant tests to maintain clean test suite
- Use second client for testing broadcast without subscription
- Add helper methods for subscribing/unsubscribing multiple channels
- Improve test reliability with proper async/await patterns and cleanup

Author: grdsdev