fix: ox crashed due to old-event-not-found-problem

I've recorded the following AI-prompts, though I'm not sure they are related to this commit:

claude-sonnet-4-6: On the main branch in the current directory, we get `AssertionError: expectation is unsupported, observation is full for search.recurrences.expanded.exception` from many servers now when running `pytest -k compat` in ~/caldav.  Reproducable by doing `cd ~/caldav ; status=good ; pytest tests/test_caldav.py::TestForServerRadicale::testCheckCompatibility || status=bad; cd ~/caldav-server-tester ; echo $status`.  CHecking out tag v3.2.0, the test passes.  There aren't that many differences between master and v3.2.0.  Please investigate.
claude-sonnet-4-6: Throw it into the current async-test-infrastructure branch.

Author: tobixen

src/caldav_server_tester/checks.py

@@ -777,6 +777,7 @@ def _run_check(self) -> None:
             event = cal.event_by_uid(uid)
         except NotFoundError:
             event = Event(cal.client, url=cal.url.join(uid + ".ics"), parent=cal)
+            event.load()
 
         vevent = event.icalendar_instance.walk("VEVENT")[0]
         original_summary = str(vevent.get("SUMMARY", ""))