[Yukang-Lian]

What problem does this PR solve?

Issue Number: None

Related PR: #66752

Problem Summary:

Backport the cumulative compaction thread-accounting fix from #66752 to branch-4.1.

When tablet global compaction lock acquisition fails, the worker exits before _cumu_compaction_thread_pool_used_threads is incremented. The deferred cleanup previously decremented the counter unconditionally, which could make it negative and distort compaction scheduling capacity.

This backport preserves the 4.1 compaction structure and guards cleanup so the counter is decremented only after it has actually been incremented. It also carries the focused C++ regression test and the Cloud fault-injection regression suite from the source fix.

Local validation:

• git diff --check
• Clang Format 16 dry-run for the changed C++ files
• clang++ -fsyntax-only for cloud_storage_engine.cpp and cloud_compaction_test.cpp using an existing 4.1 ASAN UT compile database
• Groovy 4.0.19 parse for test_cloud_cumu_compaction_global_lock_thread_count.groovy

The focused C++ and Docker regression tests were not executed locally; hosted CI remains required.

Release note

Fix Cloud cumulative compaction thread accounting after global lock acquisition failures.

Check List (For Author)

• Test

  • Regression test
  • Unit Test
  • Manual test (add detailed scripts or steps below)
  • No need to test or manual test. Explain why:
    • This is a refactor/code format and no logic has been changed.
    • Previous test can cover this change.
    • No code files have been changed.
    • Other reason

  Test cases:

  • CloudCompactionTest.cumulative_global_lock_failure_keeps_thread_count_balanced
  • test_cloud_cumu_compaction_global_lock_thread_count

[hello-stephen]

Thank you for your contribution to Apache Doris.
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Please clearly describe your PR:

1. What problem was fixed (it's best to include specific error reporting information). How it was fixed.
2. Which behaviors were modified. What was the previous behavior, what is it now, why was it modified, and what possible impacts might there be.
3. What features were added. Why was this function added?
4. Which code was refactored and why was this part of the code refactored?
5. Which functions were optimized and what is the difference before and after the optimization?

[Yukang-Lian]

run buildall

[hello-stephen]

BE Regression && UT Coverage Report

Increment line coverage 100.00% (8/8) 🎉

Increment coverage report
Complete coverage report

Category	Coverage
Function Coverage	74.35% (31366/42188)
Line Coverage	58.54% (348840/595862)
Region Coverage	55.20% (290617/526520)
Branch Coverage	56.07% (131044/233721)

[github-actions]

PR approved by anyone and no changes requested.

[github-actions]

PR approved by at least one committer and no changes requested.