Retry a presence worker that failed to start

If spawning the worker failed, the failed start stayed cached: presence
never retried and dispose() awaited the failure. A failed start now clears
itself so the next update tries again, and the error is logged rather than
thrown to the caller, as the service's own errors are.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
5188c99c00Dara Adedeji committed on 10/5/2026, 2:02:26 PMparent5145588

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