Fix race condition causing flaky hang in WritePreparedTransactionSeqnoTest

[anand1976]

Summary

• Fix a race condition in three WritePreparedTransactionSeqnoTest tests (SeqnoGoesBackwardsDuringErrorRecovery, SeqnoDiscrepancyDuringErrorRecovery, ConcurrentWritesDuringErrorRecovery) that could cause permanent hangs.
• The tests inject a filesystem error during flush via a WriteManifest sync point callback, then wait for background error recovery to complete. The bug was in the ordering of operations after recovery starts: SetFilesystemActive(true) was called before ClearCallBack, allowing a window where recovery's ResumeImpl could trigger the callback and re-disable the filesystem. This left the filesystem permanently disabled, causing all recovery retries to fail and exit without firing the RecoverSuccess sync point, leaving the test thread blocked forever.
• The fix swaps the order so ClearCallBack is called before SetFilesystemActive(true), ensuring the filesystem cannot be re-disabled by a late callback firing.

Test plan

• Stress tested with gtest_parallel (500 iterations, 32 workers, 60s timeout) with no hangs observed.
• Previously reproduced the hang at ~7% rate under stress with 15s timeout before the fix.

[meta-codesync]

@anand1976 has imported this pull request. If you are a Meta employee, you can view this in D93929251.