Stop sync poller on stopped Engine.IO sockets

[jimmystridh]

Summary

This changes the sync client callback poller so a stopped Engine.IO socket is handled explicitly instead of being ignored by the polling loop.

When RawClient::poll() returns Ok(None), Iter::next() produces Error::StoppedEngineIoSocket. Before this change, poll_callback() did not treat that error as a stop or reconnect condition, so the callback polling thread could immediately poll again forever.

Behavior

• Reconnects on StoppedEngineIoSocket when reconnect is enabled.
• Stops the callback poller on StoppedEngineIoSocket when reconnect is disabled.
• Emits the close callback once when polling stops because the underlying Engine.IO socket has closed.
• Keeps Socket.IO Disconnect packet behavior controlled by reconnect_on_disconnect.

Issue

Proposed fix for #508.

Validation

• cargo fmt --check
• cargo test -p rust_socketio stopped_engineio_socket -- --nocapture
• cargo test -p rust_socketio socketio_disconnect_obeys_reconnect_on_disconnect -- --nocapture
• cargo test -p rust_socketio ordinary_packets_keep_polling -- --nocapture