feat(realtime): integrate actor-based components into RealtimeClientV2

- Replace connection management with ConnectionStateMachine
- Replace heartbeat logic with HeartbeatMonitor
- Replace auth handling with AuthTokenManager
- Replace message routing with MessageRouter
- Update listenForMessages() to delegate to ConnectionStateMachine
- Remove old state management code (heartbeatTask, pendingHeartbeatRef, etc)
- Update push() to work with async connection property
- Clean up obsolete methods (onConnected, onDisconnected, onError, onClose, reconnect, startHeartbeating, sendHeartbeat)

Note: Existing RealtimeTests need updates as they test implementation details that have changed.

Author: grdsdev