CNDB-17453: fix flaky cleanup timing assertion in FlushFailingOnNotificationSubscriberTest

[driftx]

What does this PR fix and why was it fixed

Reset lastTimePoolNeededCleaning after successful flushes so the cleanup gap measurement doesn't span flush boundaries.

[github-actions]

Checklist before you submit for review

• This PR adheres to the Definition of Done
• Make sure there is a PR in the CNDB project updating the Converged Cassandra version
• Use NoSpamLogger for log lines that may appear frequently in the logs
• Verify test results on Butler
• Test coverage for new/modified code is > 80%
• Proper code formatting
• Proper title for each commit staring with the project-issue number, like CNDB-1234
• Each commit has a meaningful description
• Each commit is not very long and contains related changes
• Renames, moves and reformatting are in distinct commits
• All new files should contain the DataStax copyright header instead of the Apache License one

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[cassci-bot]

❌ Build ds-cassandra-pr-gate/PR-2316 rejected by Butler

---

6 regressions found
See build details here

---

Found 6 new test failures

Test	Explanation	Runs	Upstream
o.a.c.auth.CassandraRoleManagerTest.testPasswordUpdateRateLimitingPerRole (compression)	NEW	🔴⚪	0 / 30
o.a.c.concurrent.StageTimeMeasurementTest.executionAndQueueTimeAreCountedOnSubmitWithResult (compression)	REGRESSION	🔴🔴	0 / 30
o.a.c.index.sai.cql.VectorCompaction100dTest.testOneToManyCompaction[dc false] ()	NEW	🔴⚪	0 / 30
o.a.c.index.sai.cql.VectorKeyRestrictedOnPartitionTest.partitionRestrictedWidePartitionBqCompressedTest[db false] (compression)	REGRESSION	🔵🔴	0 / 30
o.a.c.index.sai.cql.VectorSiftSmallTest.testMultiSegmentBuild[ca false] ()	NEW	🔴⚪	0 / 30
o.a.c.io.util.PrefetchingRebuffererTest.testNonSequentialAccess (compression)	REGRESSION	🔴🔴	2 / 30

---

Found 4 known test failures