fix(mail): guard empty message payload in processor (PR #935 round-6)

- processor.py: raise on an empty mail_message payload (OCS data=null with a
  <400 meta) so the task dead-letters instead of indexing a near-empty
  placeholder — mirrors the nc_mail_get_message tool guard. (the round-6
  approve-gating item)
- server/mail.py: clamp limit once in nc_mail_list_messages and base has_more on
  the effective (post-clamp) limit, so a caller passing limit<=0 doesn't get a
  misleading count.
- server/mail.py: note in nc_mail_get_message that attachments with id=null are
  inline body parts and can't be fetched via nc_mail_get_attachment.
- tests: add the first-time-missing incremental scanner case (enters the grace
  period, nothing queued/deleted).

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

nextcloud_mcp_server/vector/processor.py

@@ -855,6 +855,13 @@ async def _index_document(
             if not is_valid_nextcloud_doc_id(doc_task.doc_id):
                 raise ValueError(f"Invalid mail_message doc_id: {doc_task.doc_id!r}")
             message = await nc_client.mail.get_message(int(doc_task.doc_id))
+            # An empty payload (OCS data=null with a <400 meta) would otherwise
+            # index a useless near-empty placeholder; fail loudly so the task
+            # dead-letters instead of corrupting the index.
+            if not message:
+                raise ValueError(
+                    f"mail_message {doc_task.doc_id!r} returned an empty payload"
+                )
             content = build_mail_content(message)
 
             subject = message.get("subject") or ""