[0.9.x][Hotfix] Force ReachableFileCleanup when snapshots exist outside main ancestry (backport #4231)

[j1wonpark]

Backport of #4231 to 0.9.x

This is a clean cherry-pick of #4231 (d8f37934) onto the 0.9.x release branch.

Why are the changes needed?

#4231 is a data-loss fix. Iceberg's auto-selected IncrementalFileCleanup
can silently truncate its ancestor walk when a parent snapshot is missing, and
delete data files that the current snapshot still references. This PR forces the
safe ReachableFileCleanup only when snapshots exist outside the main ancestry;
healthy tables are unchanged.

The fix is currently on master only. Since 0.9.0 is still in the release-candidate
stage, including this fix in the release avoids shipping a known data-loss path.

Brief change log

• Force ReachableFileCleanup in IcebergTableMaintainer when snapshots exist
  outside the main ancestry (ReachableFileCleanupBridge exposes the safe path).
• Add TestExpireSnapshotsKeepReferencedFiles covering the regression.

Verification

• Cherry-picks cleanly onto 0.9.x with no conflicts.
• Same code and tests as the already-merged [ams] Force ReachableFileCleanup when snapshots exist outside main ancestry #4231 on master.

cc the 0.9.0 release manager — flagging for inclusion in the next RC.

Original PR: #4231

[codecov-commenter]

Codecov Report

❌ Patch coverage is 0% with 22 lines in your changes missing coverage. Please review.
✅ Project coverage is 29.90%. Comparing base (19d6a71) to head (d0c29a9).
⚠️ Report is 9 commits behind head on 0.9.x.

Files with missing lines	Patch %	Lines
...ats/iceberg/maintainer/IcebergTableMaintainer.java	0.00%	19 Missing ⚠️
...org/apache/iceberg/ReachableFileCleanupBridge.java	0.00%	3 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##              0.9.x    #4241      +/-   ##
============================================
+ Coverage     29.80%   29.90%   +0.09%     
- Complexity     4285     4319      +34     
============================================
  Files           677      678       +1     
  Lines         54988    55052      +64     
  Branches       7012     7032      +20     
============================================
+ Hits          16391    16465      +74     
+ Misses        37376    37354      -22     
- Partials       1221     1233      +12     

Flag	Coverage Δ
core	29.90% <0.00%> (+0.09%)	⬆️

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[j1wonpark]

@czy006 @xxubai could you please take a look at this backport?

#4231 is a data-loss fix that is currently on master only. The cherry-pick is clean (no conflicts) and the code/tests are identical to the already-merged #4231.

Thanks!