test: add comprehensive unit tests for 100% coverage

- Add 10 new unit tests covering all previously untested code paths
- Test connection pool integration with push notifications
- Test connection health check integration
- Test Conn type push notification methods
- Test cloned client push notification preservation
- Test PushNotificationInfo structure validation
- Test edge cases and error scenarios
- Test custom processor integration
- Test disabled push notification scenarios

Total coverage now includes:
- 20 existing push notification tests
- 10 new comprehensive coverage tests
- All new code paths from connection pool integration
- All Conn methods and cloning functionality
- Edge cases and error handling scenarios

Author: ndyakov