[branch-4.1][fix](paimon) Fix external regression failures

[suxiaogang223]

What problem does this PR solve?

The branch-4.1 Paimon partition metadata backport makes partition-only scans produce an empty physical-file projection. The JNI scanner parsed empty required_fields as one empty field, mapped it to index -1, and failed during MTMV ANALYZE with Index -1 out of bounds.

The runtime-filter regression also set enable_file_scanner_v2, which is unavailable on branch-4.1, so the suite failed while setting and unsetting that variable.

What changed?

• Treat empty Paimon JNI projections as zero-field scans.
• Append virtual row counts so BE can fill partition columns from path metadata.
• Validate required field/type lengths and projection lookup.
• Add empty-projection constructor coverage.
• Remove the unsupported scanner variable from the branch-4.1 regression.

Validation

• ./build.sh --fe
• mvn -pl be-java-extensions/paimon-scanner -am -Dtest=PaimonJniScannerTest -Dsurefire.failIfNoSpecifiedTests=false test
• git diff --check

[hello-stephen]

Thank you for your contribution to Apache Doris.
Don't know what should be done next? See How to process your PR.

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

[hello-stephen]

FE UT Coverage Report

Increment line coverage `` 🎉
Increment coverage report
Complete coverage report

[suxiaogang223]

Closing as superseded by #65559, which has landed on branch-4.1 and already contains the Paimon JNI empty-projection handling, virtual row data, and constructor test. It also backports FileScannerV2, so removing enable_file_scanner_v2 is no longer appropriate.