Two issues in MaintNotificationsConnectionPoolMixin:

1. _get_free_connections and _get_in_use_connections return direct references to internal collections

The lock is held only during return, but callers iterate the returned collection outside the lock (e.g., in re_auth_callback). Another thread mutating the pool can cause RuntimeError: list/set changed size during iteration. The BlockingConnectionPool already does this correctly by returning copies.

Fixed by returning list(self._available_connections) and set(self._in_use_connections).

2. Sync _mock is async def but called from sync context

re_auth_callback is a sync method that passes _mock as a failure callback to the sync Retry.call_with_retry. Calling an async def from sync context returns an unawaited coroutine. Removed the async keyword since this is intentionally a no-op callback.