test: add fire_and_forget to API chat webhook stub (#4383)

tests/test_api_chat_security.py

@@ -219,6 +219,9 @@ class _WebhookManager:
     async def fire(self, event, payload):
         return None
 
+    def fire_and_forget(self, event, payload):
+        return None
+
 
 def _install_sync_chat_stubs(monkeypatch):
     # FastAPI checks for python_multipart at import time when Form is used;