fix(mail): address PR #935 round-2 review

- server/mail.py: guard nc_mail_get_message against an empty OCS payload so
  MailMessage(**{}) can't raise an uncaught ValidationError (returns a clean
  'not found' instead).
- server/mail.py: cap inlined attachment content at MAX_ATTACHMENT_CONTENT_BYTES
  (5 MiB), replacing oversized bodies with a sentinel so a large attachment
  can't blow up the MCP response.
- client/mail.py: harden the OCS meta statuscode parse against a non-numeric
  value (treat as success) instead of letting int() raise an uncaught
  ValueError.
- scanner.py: log the newest-N cap hit once per (user, mailbox) at info level
  (discoverable without flooding multi-tenant logs on every scan tick).
- tests: add incremental-sync scanner cases (new message queued, reappeared
  message clears grace, deletion after grace expiry).

Deferred (tracked, card #376): include doc_type in the _potentially_deleted
grace-period key — a pre-existing cross-cutting collision the reviewer flagged
as a follow-up, not a blocker.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

nextcloud_mcp_server/vector/scanner.py

@@ -1362,6 +1362,12 @@ async def scan_news_items(
 # wanted, at the cost of more embedding work.
 MAIL_SCAN_MAX_PER_MAILBOX = 100
 
+# Per-process record of (user_id, mailbox_id) for which the newest-N cap has
+# already been logged, so the "older mail not indexed" notice is emitted once at
+# info level (discoverable) rather than on every scan tick (which would flood
+# multi-tenant logs).
+_mail_cap_logged: set[tuple[str, int]] = set()
+
 
 async def scan_mail_messages(
     user_id: str,
@@ -1460,10 +1466,15 @@ async def scan_mail_messages(
                 )
                 continue
 
-            if len(messages) >= MAIL_SCAN_MAX_PER_MAILBOX:
-                logger.debug(
+            cap_key = (user_id, mailbox_id)
+            if len(messages) >= MAIL_SCAN_MAX_PER_MAILBOX and cap_key not in (
+                _mail_cap_logged
+            ):
+                _mail_cap_logged.add(cap_key)
+                logger.info(
                     "[SCAN-%s] Mailbox %s hit the newest-%s cap; older messages "
-                    "are not indexed",
+                    "are not indexed (set MAIL_SCAN_MAX_PER_MAILBOX higher for "
+                    "deeper history)",
                     scan_id,
                     mailbox_id,
                     MAIL_SCAN_MAX_PER_MAILBOX,