[opt](paimon) support variant access path in paimon jni reader

[zhangstar333]

What problem does this PR solve?

Problem Summary:

support read variant sub path in jni reader using access path

|   0:VPAIMON_SCAN_NODE(87)                                                     |
|      table: test_variant.variant_db.variant_shredded                          |
|      inputSplitNum=1, totalFileSize=0, scanRanges=1                           |
|      partition=1/0                                                            |
|      cardinality=2, numNodes=1                                                |
|      nested columns:                                                          |
|        payload:                                                               |
|          origin type: variant                                                 |
|          all access paths: [payload.name]                                     |
|      pushdown agg=NONE                                                        |
|      paimonNativeReadSplits=0/1                                               |
|      predicatesFromPaimon: NONE                                               |
|      final projections: id[#0], CAST(element_at(payload[#1], 'name') AS text) |
|      final project output tuple id: 1

Release note

None

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

• Behavior changed:

  • No.
  • Yes.

• Does this need documentation?

  • No.
  • Yes.

Check List (For Reviewer who merge this PR)

• Confirm the release note
• Confirm test cases
• Confirm document
• Add branch pick label

[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]

Reviewed at b148688 against the latest branch-4.1. I did not find a blocking correctness issue. The three-way merge also preserves the recently merged fix that unsets parquet.variant.shreddingSchema before writing unshredded files.

Non-blocking test coverage suggestions:

• Add an end-to-end JNI regression for a nested shredded object path such as profile.city. Nested extraction is currently covered only by Java unit tests.
• Add a JNI regression for an unsupported numeric or array path such as payload[tags][0], verifying that it correctly falls back to reading the full Variant.
• Add feature-gate tests for Variant nested inside ARRAY, MAP, and STRUCT; the implementation checks these types recursively, while the current test covers only a top-level Variant.

[Gabriel39]

run buildall

[zhangstar333]

run buildall

[hello-stephen]

FE Regression Coverage Report

Increment line coverage 93.75% (15/16) 🎉
Increment coverage report
Complete coverage report

[zhangstar333]

run buildall

[hello-stephen]

FE UT Coverage Report

Increment line coverage 54.55% (6/11) 🎉
Increment coverage report
Complete coverage report

[hello-stephen]

BE UT Coverage Report

Increment line coverage 100.00% (22/22) 🎉

Increment coverage report
Complete coverage report

Category	Coverage
Function Coverage	58.33% (24737/42412)
Line Coverage	42.60% (249476/585579)
Region Coverage	38.64% (197085/510025)
Branch Coverage	40.06% (90280/225384)