Change _lock from Lock to RLock and move the start_enrich call inside the
lock-held block so that enqueue_enrich cannot interleave between clearing
_pending_enrich and starting the enrichment scan. This prevents a concurrent
enqueue_enrich from stealing the IDLE slot and causing the drained payload
to be silently dropped.
Add tests covering:
- pending enrich runs after scan completes
- enqueue during drain does not lose work
- concurrent enqueue during drain is queued for the next cycle
Amp-Thread-ID: https://ampcode.com/threads/T-019cfe02-5710-7506-ae80-34bf16c0171a
Co-authored-by: Amp <amp@ampcode.com>
