CNDB-15697: Fix flush error handling for shallow SSTable creation

[driftx]

Add defensive checks in SSTableWriter and SSTableReader to handle closed channels and null file handles when creating shallow readers during flush error recovery.

SSTableWriter: Catch IllegalStateException on file pointer access
SSTableReader: Handle null dfile in bytesOnDisk()

[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

[djatnieks]

I think the test regressions are probably unrelated - I'll trigger another build to check

[sonarqubecloud]

Quality Gate failed

Failed conditions
18.2% Coverage on New Code (required ≥ 80%)

See analysis details on SonarQube Cloud

[cassci-bot]

❌ Build ds-cassandra-pr-gate/PR-2067 rejected by Butler

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7 regressions found
See build details here

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Found 7 new test failures

Test	Explanation	Runs	Upstream
o.a.c.cql3.validation.operations.AggregationQueriesTest.testAggregationQueryShouldNotTimeoutWhenItExceedesReadTimeout (compression)	REGRESSION	🔴🔴	2 / 11
o.a.c.db.ColumnFamilyStoreTest.testSnapshotCreationAndDeleteEmptyTable ()	NEW	🔴⚪	1 / 11
o.a.c.distributed.test.hostreplacement.HostReplacementTest.replaceAliveHost	REGRESSION	🔵🔴	0 / 11
o.a.c.distributed.test.hostreplacement.HostReplacementTest.replaceDownedHost	REGRESSION	🔵🔴	0 / 11
o.a.c.distributed.test.hostreplacement.HostReplacementTest.retryingFailedReplaceWithNodeInHibernateState	REGRESSION	🔵🔴	0 / 11
o.a.c.distributed.test.hostreplacement.HostReplacementTest.seedGoesDownBeforeDownHost	REGRESSION	🔵🔴	0 / 11
o.a.c.distributed.test.repair.ForceRepairTest.forceWithDifference ()	NEW	🔴⚪	6 / 11

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Found 5 known test failures

[djatnieks]

After the second PR build the tests look good to me.