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.