fix: restore missing docstring on test_event_setter_none

The return-type annotation added in the branch accidentally dropped the
docstring that was on test_event_setter_none. Restore it.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: jacalata

test/test_webhook.py

@@ -92,6 +92,7 @@ def test_request_factory():
 
 
 def test_event_setter_none() -> None:
+    """Setting event to None should store None without crashing."""
     item = WebhookItem()
     item.event = "datasource-updated"
     assert item.event == "datasource-updated"