Intermittent test failure: should not allow parallel updates to the same Entitys

https://github.com/getodk/central-backend/actions/runs/13279371367/job/37074603029?pr=1394

```
1) Entities API
PATCH /datasets/:name/entities/:uuid
should not allow parallel updates to the same Entity:
Error: Timeout of 2000ms exceeded. For async tests and hooks, ensure "done()" is called; if returning a Promise, ensure it resolves. (/home/runner/work/central-backend/central-backend/test/integration/api/entities.js)
at listOnTimeout (node:internal/timers:594:17)
at process.processTimers (node:internal/timers:529:7)
```

* there's a chance this failure is from a deadlock, as that seems close to the subject of the test
* maybe just a slow test? probably not - when retried, this test took 112ms

Start with the failing integration test in test/integration/api/entities.js and the linked GitHub Actions run. Investigate why PATCH requests for the same entity time out, including whether the failure is a deadlock or an intermittent slow test. Done means the test no longer intermittently times out and still verifies that parallel updates are rejected.

Written by the indexing model from the issue text.