fix(tests): add missing json import in msgraph webhook test fixture

The salvaged #25296 fixture's _FakeRequest.read() calls json.dumps but the
test module never imported json — the NameError was swallowed by the
handler's generic except → 400, failing 10 payload tests.

tests/gateway/test_msgraph_webhook.py

@@ -1,6 +1,7 @@
 """Tests for the Microsoft Graph webhook adapter."""
 
 import asyncio
+import json
 
 import pytest