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/vector/scanner.py

@@ -1549,6 +1549,16 @@ async def scan_mail_messages(
                         needs_indexing = True
                     elif existing_metadata.get("modified_at", 0) < modified_at:
                         needs_indexing = True
+                    elif existing_metadata.get("status") == "failed":
+                        # A permanent processing failure — don't re-queue an
+                        # unchanged message that will just fail again; the
+                        # modified_at branch above retries once it changes
+                        # (mirrors the file scanner's failed-placeholder guard).
+                        logger.debug(
+                            "Skipping mail message %s: previous processing "
+                            "failed permanently",
+                            doc_id,
+                        )
                     elif existing_metadata.get("is_placeholder", False):
                         queued_at = existing_metadata.get("queued_at", 0)
                         placeholder_age = time.time() - queued_at