feat: logging for semaphore

[phact]

This pull request introduces improved tracking and logging of worker slot contention in the TaskService, making it easier to diagnose and monitor concurrency bottlenecks during file processing. The main changes include adding internal accounting for active and queued workers, logging when tasks wait for a worker slot, and updating tests to verify this behavior.

Worker slot accounting and logging:

• Added _worker_stats_lock, _active_workers, and _queued_workers to TaskService for accurate tracking of worker slot usage. Introduced _acquire_processing_slot and _release_processing_slot async methods to handle slot acquisition, release, and contention logging.
• Updated the file processing flow to use the new slot acquisition/release methods, ensuring that tasks log when they are queued and when they wait for a worker slot. [1] [2] [3]

Testing improvements:

• Added an async test test_logs_when_waiting_for_worker_slot to verify that contention is properly logged when tasks must wait for a worker slot.

Minor cleanup:

• Removed an unnecessary blank line in the shutdown exception handler for consistency.

[edwinjosechittilappilly]

LGTM