Task handlers were swallowing exceptions, causing operations to be
marked as completed even when they failed. This prevented the retry
logic in execute_task() from working and led to accumulation of
pending operations that never completed.
Fixed handlers:
- _handle_batch_retain: remove try/except wrapper
- _handle_access_count_update: remove try/except wrapper
- _handle_regenerate_observations: remove outer try/except, keep
inner one for individual entity failures
904ea4de24