[enhancement](variant) Support deep Variant object leaf projection

[Gabriel39]

What problem does this PR solve?

Issue Number: None

Related PR: None

Problem Summary:

File Scanner V2 carried Variant access paths as a list of string segments, but the BE Parquet mapper only projected one top-level shredded leaf. Deep object paths therefore fell back even when every nested typed_value wrapper was physically available. The FE access-path comparator also joined segments with dots, so an object key such as a.b could collide with the nested path a/b before the paths reached BE.

This change reuses the existing access-path list without any Thrift or Proto change. It compares FE paths segment by segment, merges sibling and prefix paths through the shared BE access-path tree, and traverses arbitrary-depth object typed_value wrappers for Iceberg and Paimon native Parquet scans. Numeric segments, repeated ancestors, missing leaves, unshredded files, and ambiguous physical leaf identities conservatively fall back to the complete Variant root. Numeric array indexes remain unsupported with a TODO for typed path segments.

Release note

Support arbitrary-depth object-only Variant leaf projection for Iceberg and Paimon native Parquet scans.

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. Iceberg and Paimon native Parquet scans can physically project deeply shredded Variant object leaves while preserving conservative fallback behavior.

• 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]

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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.
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4. Which code was refactored and why was this part of the code refactored?
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[Gabriel39]

run buildall

[Gabriel39]

/review

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[Gabriel39]

run buildall

[Gabriel39]

/review

[github-actions]

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[hello-stephen]

BE Regression && UT Coverage Report

Increment line coverage 100.00% (13/13) 🎉

Increment coverage report
Complete coverage report

Category	Coverage
Function Coverage	73.79% (30529/41373)
Line Coverage	57.77% (336077/581714)
Region Coverage	54.63% (279612/511865)
Branch Coverage	55.58% (125296/225422)

[hello-stephen]

FE Regression Coverage Report

Increment line coverage 100.00% (11/11) 🎉
Increment coverage report
Complete coverage report