branch-4.1: [fix](paimon) Support Variant in native reader

[Gabriel39]

What's changed

• Backport the projected shredded Variant exchange and TopN fixes from [fix](variant) Preserve projected shredded states across exchange #66446.
• Let the Paimon native Parquet reader interpret schema-matched, unannotated Variant carriers without misclassifying ordinary structs.
• Preserve native projection and predicate pruning for shredded Variant leaves, including mixed shredded and unshredded files.
• Keep lazy Variant reconstruction counters alive after scanner profile destruction.
• Add schema, reader, lifecycle, mixed-layout, native/JNI, subpath, null, array, and predicate coverage.

Tests

• Focused ASAN unit test binary: 187 tests passed.
• FE Release build and Checkstyle passed.
• BE Release compilation passed.
• test_paimon_catalog_variant regression suite passed with both JNI and native reads.

[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?

[Gabriel39]

run buildall

[Gabriel39]

/review

[github-actions]

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Error: All Codex review accounts are usage-limited; earliest retry is 2026-08-08T03:32:00Z.
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[Gabriel39]

run buildall

[Gabriel39]

/review

[github-actions]

Codex automated review failed and did not complete.

Error: All Codex review accounts are usage-limited; earliest retry is 2026-08-08T03:32:00Z.
Workflow run: https://github.com/apache/doris/actions/runs/31011912192

Please trigger /review again after that time.

[hello-stephen]

BE Regression && UT Coverage Report

Increment line coverage 80.76% (684/847) 🎉

Increment coverage report
Complete coverage report

Category	Coverage
Function Coverage	73.77% (30484/41323)
Line Coverage	57.72% (335369/580978)
Region Coverage	54.59% (279011/511145)
Branch Coverage	55.54% (125003/225066)