Bounded cleanup owner for token managers left by failed provider acquisition.

This owner serializes adoption and gives each manager exactly one linked cleanup worker, including across concurrent or repeated adoption attempts. The worker retries bounded close calls with capped exponential backoff. Consecutive close timeouts have a finite cleanup-slot budget and reclaim a manager abnormally when it is exhausted. A persistence failure resets that timeout streak and remains retryable without a cleanup deadline. If this owner restarts, OTP terminates the linked workers, which in turn terminate their linked managers rather than leaving unreachable credential owners alive. Provider cleanup uses adopt/2 to confirm both worker ownership and removal from the registrar's force-close set as one caller-visible handoff. Worker registration is serialized here, while registrar calls run as deduplicated asynchronous operations so a stalled scope cannot block unrelated cleanup. Callers add no competing timeout; each registrar handoff remains bounded by its local five-second limit.