Flaky test ScheduledRetryingExecutorTest.testSuccessWithFailuresGetAttempt

```
Error: Errors:
Error: ScheduledRetryingExecutorTest.testSuccessWithFailuresGetAttempt:172->AbstractRetryingExecutorTest.assertFutureSuccess:404 � Execution com.google.api.gax.retrying.FailingCallable$CustomException
```

See the [error log](https://github.com/googleapis/google-cloud-java/actions/runs/27588770400/job/81564863466) for details.

Start with ScheduledRetryingExecutorTest.testSuccessWithFailuresGetAttempt and the referenced AbstractRetryingExecutorTest.assertFutureSuccess failure, then reproduce the test and inspect the linked error log. Done means the test reliably passes while preserving the expected attempt count and success behavior.

索引モデルが issue の本文から書いたものです。