polish(mail): consistency guards (PR #935 round-7)

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>

nextcloud_mcp_server/search/context.py

@@ -834,6 +834,10 @@ async def _fetch_document_text(
             # Reconstruct full content via the shared helper so chunk offsets
             # match what the processor indexed (single source of truth).
             message = await nc_client.mail.get_message(int(doc_id))
+            # Empty payload (OCS data=null with a <400 meta) -> skip context
+            # expansion, mirroring the processor's index-time guard.
+            if not message:
+                return None
             return build_mail_content(message)
         else:
             logger.warning("Unsupported doc_type for context expansion: %s", doc_type)