refactor: remove unused async connect path and simplify response handling

[jy-tan]

Summary

Removes dead code from ProtobufCommunicator which has potential issues with blocking the event loop and message multiplexing. The async connect() method and its helpers were never used because the SDK exclusively uses connect_sync().

Changes

Removed:

• async def connect() - Unused async connection method with blocking socket calls inside an async function
• async def _send_connect_message() - Helper only called by async connect
• async def _receive_connect_response() - Helper only called by async connect
• Dead "no background reader" branch in _receive_response() that used blocking socket reads and dropped non-matching messages (no multiplexing)

Modified:

• disconnect() - Changed from async to sync since it only calls _cleanup()
• _receive_response() - Simplified to always delegate to _wait_for_response_async() via the background reader thread
• drift_sdk.py - Updated to call disconnect() directly instead of via asyncio.run()

Context

The removed async connect() path had two issues:

1. Blocking socket reads in async function - Could stall the event loop
2. No message multiplexing - Messages not matching the current request_id were silently discarded

These were latent bugs since the code path was never reached (connect_sync() is always used), and we probably won't need the async path, but removing them eliminates potential confusion and reduces maintenance burden.

[cubic-dev-ai]

No issues found across 2 files