Non-blocking consistency fixes:
- scanner.py: skip re-queuing a mail_message whose placeholder status is
"failed" (mirrors the file scanner's permanent-failure guard); the
modified_at branch still retries once the message changes.
- search/context.py: return None on an empty get_message payload during context
expansion, mirroring the processor's index-time empty-payload guard.
- tests: cover the non-numeric mailbox_id fail-open branch in the verifier.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
