[branch-4.1](variant) Fix compaction failure on no-key table with variant column uid=0 #62571

[csun5285]

pick from master #62571#62632

[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?

[csun5285]

run buildall

[hello-stephen]

BE UT Coverage Report

Increment line coverage 66.67% (4/6) 🎉

Increment coverage report
Complete coverage report

Category	Coverage
Function Coverage	53.21% (19960/37513)
Line Coverage	36.68% (187985/512440)
Region Coverage	33.03% (146151/442506)
Branch Coverage	34.13% (63955/187388)

[hello-stephen]

BE Regression && UT Coverage Report

Increment line coverage 100.00% (6/6) 🎉

Increment coverage report
Complete coverage report

Category	Coverage
Function Coverage	71.41% (26223/36720)
Line Coverage	54.31% (277359/510723)
Region Coverage	51.52% (230017/446450)
Branch Coverage	52.96% (99514/187914)

[csun5285]

/review

[github-actions]

Reviewed the backport against the changed branch-4.1 codepaths and the upstream fixes.

• Correctness: Pass. The uid == 0 case is handled consistently across extracted-column classification, compaction schema merge, scanner access-path routing, and variant stats collection.
• Compatibility: Pass. The follow-up has_parrent_column_unique_id() change removes the uint32 >= 0 compile hazard while preserving the intended runtime behavior for uid == 0.
• Test coverage: Pass. The BE unit test exercises compaction on a no-key variant table with root unique_id = 0, and the new regression test validates query results before and after cumulative compaction.
• Performance and operational risk: Pass. The changes are narrowly scoped condition fixes with no material algorithmic impact.
• Blocking issues: None found.

Residual risk: there is still no targeted test for scanner access-path pruning on a uid == 0 variant column, but I did not find a correctness issue in the current implementation.