branch-4.0: [fix](paimon) preserve partition metadata across splits

[suxiaogang223]

What problem does this PR solve?

Paimon partition metadata was generated only when runtime partition pruning was enabled. Some count/native/JNI splits therefore reached BE without stable partition values. BE also reused a partition-prune block across splits but inserted replacement columns, corrupting the block layout.

Changes

• Always attach ordered Paimon partition metadata to every data split.
• Generate aligned columns_from_path_keys, columns_from_path, and columns_from_path_is_null arrays in FE.
• Pass explicit null markers from FileScanner to Parquet/ORC readers.
• Preserve NULL, empty string, and literal \N semantics.
• Replace partition columns in the reusable prune block instead of inserting new columns.
• Preserve the partition-key case behavior already present on the latest branch-4.0 baseline.
• Add FE/BE unit coverage and a three-partition static IN regression case.

This is a focused backport of the relevant #62821 behavior adapted to the pre-#62306 reader architecture.

Validation

• clang-format 16
• git diff --check
• FE checkstyle for touched Java files
• Full FE/BE tests will run in CI; local FE UT compilation is blocked by missing generated Thrift Java classes in this worktree.

[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?

[suxiaogang223]

run buildall

[suxiaogang223]

run buildall

[hello-stephen]

FE UT Coverage Report

Increment line coverage 60.00% (21/35) 🎉
Increment coverage report
Complete coverage report

[hello-stephen]

BE UT Coverage Report

Increment line coverage 53.16% (42/79) 🎉

Increment coverage report
Complete coverage report

Category	Coverage
Function Coverage	53.53% (19572/36562)
Line Coverage	36.60% (183171/500435)
Region Coverage	33.18% (142467/429426)
Branch Coverage	34.02% (61563/180981)

[suxiaogang223]

run buildall

[hello-stephen]

FE UT Coverage Report

Increment line coverage 60.00% (21/35) 🎉
Increment coverage report
Complete coverage report

[github-actions]

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

[github-actions]

PR approved by anyone and no changes requested.