OAK-12121 : add regression tests for offline compaction not persisting gc.log

[rishabhdaim]

Summary

• Adds OfflineCompactionGcLogTest — three regression tests that directly expose the bug where offline compaction (compactFull() + cleanup() as separate calls) never writes a gc.log entry
• All three tests fail against the unfixed code and pass once the fix retaining CompactionResult across the two calls is applied

Changes

• oak-segment-tar/src/test/java/.../file/OfflineCompactionGcLogTest.java (new file)
  • testOfflineCompactionPersistsGcLog — basic offline compaction scenario: verifies gc.log has a valid entry with non-empty root id and incremented GC generation
  • testCleanupFirstCompactOkCleanupGcLogWritten — cleanup → compact → cleanup: pre-compaction cleanup must not write gc.log; only the post-compaction cleanup should
  • testCompactOkCleanupCleanupGcLogHasExactlyOneEntry — compact → cleanup → cleanup: first cleanup writes, second must not write a duplicate

Test Plan

• All three new tests fail on unfixed code (regression tests)
• All three new tests pass once the fix is applied

Links

• JIRA: https://issues.apache.org/jira/browse/OAK-12121

[rishabhdaim]

@lweitzendorf could you please review these test cases.

[reschke]

...grom the perspective that it reproduces the issue, so we can proceed with the fix.

[lweitzendorf]

The test cases seem reasonable. We might want to rethink whether GCJournal persistance should be handled as part of the clean up longer term, but that doesn't need to be part of the more immediate fix and these tests by extension.