background refresh thread cannot be stopped, so discarded instances keep a thread alive and keep logging in #10253

Problem description

Each KeyAuth instance starts a daemon thread (token_refresh_task) that loops forever. There's no way to stop it: no public close(), the loop has no exit condition.
We're using the to-be-released pubsub-sdk-python that inherits this in long-running processes (Airflow Trigger) that create more than one KeyAuth over their lifetime (e.g., key rotation), every discarded instance leaks a thread that keeps requesting tokens.

Proposed solution

Add a public, idempotent close()

def close(self) -> None:
    """Stop the background refresh thread and executor. Safe to call more than once."""
    self._stop.set()
    self.executor.shutdown(wait=False, cancel_futures=True)
    atexit.unregister(self.__shutdown)


Modified loop:
def __start_token_refresh_task(self):
    def token_refresh_task():
        while not self._stop.wait(self.TOKEN_EXPIRY_CHECK_INTERVAL.total_seconds()):

Additional information

Many thanks in advance!