revert(worker): rely on visibility-timeout recovery (staging)

[suguanYang]

Summary

Staging cherry-pick of the reviewed cleanup in #284. The patch IDs are identical.

• Remove the ineffective bounded gevent cancellation introduced by fix(worker): bound gevent warm shutdown #282.
• Retain the independent Redis visibility watchdog and 4,500-second visibility contract.
• Retain billing/idempotency behavior and document the accepted interruption fallback in the worker bootstrap.

Validation

• 14 passed across worker shutdown, visibility recovery, watchdog, bootstrap, and health contracts on the current staging base
• Ruff passed
• Patch ID matches revert(worker): rely on visibility-timeout recovery #284

Merge after #284. Merging this PR will trigger the normal staging deployment.