The run loop retried with a raw ExponentialBackOff, so a backoff sleep
ignored context cancellation. Now that ConnectClient.Stop waits for the
run loop to exit, a cancel landing during a sleep would block Stop for
the full interval (up to MaxInterval). Wrap the backoff with the run
context so Retry returns promptly on cancel; the retry budget itself
(MaxElapsedTime) is unchanged.
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