Cleanup event handlers on disconnect

[DTCurrie]

Event listeners for WebRTC connection state changes were not cleared during reconnections, causing handlers to accumulate with every reconnect. This resulted in old handlers firing on stale connections and potential race conditions due to duplicate event handling.

This PR implements event listener lifecycle management in RobotClient by adding class properties to store handler references (onICEConnectionStateChange, onDataChannelClose, onTrack), creating the cleanupEventListeners method to clear handler references, updating connect to call cleanup before registering new handlers, and updating disconnect to clean up handlers before closing connections.

It also introduces test coverage for client.ts, though the tests are currently limited just to event handling.