test(discord): annotate make_attachment content_type as Optional[str]

Copilot review: the helper accepted None in one test but was annotated str.
Matches actual usage where no-content-type attachments are a tested scenario.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

tests/gateway/test_discord_document_handling.py

@@ -9,6 +9,7 @@ import os
 import sys
 from datetime import datetime, timezone
 from types import SimpleNamespace
+from typing import Optional
 from unittest.mock import AsyncMock, MagicMock, patch
 
 import pytest
@@ -111,7 +112,7 @@ def adapter(monkeypatch):
 def make_attachment(
     *,
     filename: str,
-    content_type: str,
+    content_type: Optional[str],
     size: int = 1024,
     url: str = "https://cdn.discordapp.com/attachments/fake/file",
 ) -> SimpleNamespace: