Drain local workers by working off remaining jobs on SIGTERM

[svkrieger]

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A short explanation of the proposed change:

This adds LocalWorkerDrainPlugin, which replaces the SIGTERM handler for local workers only. Instead of stopping, the worker sets exit_on_complete so it finishes all remaining jobs in the queue before exiting, avoiding dangling PROCESSING_UPLOAD/PENDING states across restarts.

The plugin is a no-op for generic workers and named clock queues, which retain the default SIGTERM behaviour.

An explanation of the use cases your change solves

Local workers handle jobs that require local filesystem access (package, droplet, and buildpack uploads). When a local worker receives SIGTERM during drain, the default delayed_job handler calls stop(), which exits after the current job but leaves remaining queued jobs unprocessed until the worker restarts.

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