harden(mail): address PR #935 round-5 review

No blockers raised; residual cleanup:
- processor.py: guard int(doc_task.doc_id) with is_valid_nextcloud_doc_id in
  the mail_message branch (consistent with search/context.py + the verifier).
- mail metadata symmetry: store `bcc` in file_metadata and the Qdrant payload
  alongside cc (build_mail_content already emits a Bcc: line).
- server/mail.py: extract _cap_attachment_content helper (byte-accurate cap)
  and unit-test it (small/None/oversized/multibyte).
- client/mail.py: give the synthetic OCS-error Response an explicit empty body;
  add a test that a traversal-style attachment_id is percent-encoded.
- models/mail.py: clarify ListMessagesResponse.total_count is the page count,
  not the mailbox total.

Deferred (Deck #376): _potentially_deleted doc_type-in-key. It's pre-existing
and spans ~30 sites across the notes/news/deck/file/mail scanners (whose
deletion paths have no unit coverage), so it belongs in its own focused PR
rather than expanding this mail PR's blast radius into other doc types.

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

tests/client/mail/test_mail_api.py

@@ -196,6 +196,25 @@ async def test_get_attachment_unwraps_json(mocker):
     assert args == ("GET", "/ocs/v2.php/apps/mail/api/message/100/attachment/1.2")
 
 
+async def test_get_attachment_url_encodes_attachment_id(mocker):
+    """A traversal-style attachment_id is percent-encoded in the URL path."""
+    mock_response = _ocs_response({"name": "x", "content": "y"})
+    mock_client = mocker.AsyncMock(spec=httpx.AsyncClient)
+    mock_make_request = mocker.patch.object(
+        MailClient, "_make_request", return_value=mock_response
+    )
+
+    client = MailClient(mock_client, "testuser")
+    await client.get_attachment(100, "../../evil")
+
+    args, _ = mock_make_request.call_args
+    # The "/" and ".." are encoded, so they can't escape the attachment path.
+    assert args == (
+        "GET",
+        "/ocs/v2.php/apps/mail/api/message/100/attachment/..%2F..%2Fevil",
+    )
+
+
 async def test_empty_data_returns_empty_list(mocker):
     """A null ocs.data payload degrades to an empty list for list endpoints."""
     mock_response = _ocs_response(None)