Flaky test: ExpirationCacheTest.testGracePeriod

## Flaky Test

* **Test Name:** ` ExpirationCacheTest.testGracePeriod`
* **Link:** https://github.com/JanusGraph/janusgraph/blob/ac000a4e5425abcd35d20bef063288b06f74438c/janusgraph-test/src/test/java/org/janusgraph/diskstorage/cache/ExpirationCacheTest.java#L101
* **Branch:** `master` (PR: #3310)
* **Notes:**

### Stack Trace

```
Error: ExpirationCacheTest.testGracePeriod:104->testGracePeriod:144 expected: <0> but was: <1>
```

Start with janusgraph-test/src/test/java/org/janusgraph/diskstorage/cache/ExpirationCacheTest.java around line 101 and run ExpirationCacheTest.testGracePeriod repeatedly. Compare the failure at lines 104–144 with the test's timing and expiration behavior. Done means the test passes reliably without the expected 0-versus-1 failure.

Written by the indexing model from the issue text.