fix flaky event persister test

### Bug summary

see https://github.com/PrefectHQ/prefect/actions/runs/17073811360/job/48409684694
```
FAILED tests/events/server/storage/test_event_persister.py::test_trims_messages_periodically - assert 3 == 2
```

### Version info

```Text
main
```

### Additional context

_No response_