branch-4.2: [fix](external) Keep sub-field predicate access paths

[hubgeter]

What problem does this PR solve?

Issue Number: None

Related PR: #68214

Problem Summary:

retainPredicatePathsInFinalAllAccessPaths, added by #68214, removes every predicate access path that is not literally one of the final all access paths. That is what NULL/OFFSET paths need, because they are stripped from the all paths on purpose, but it also removes ordinary sub-field paths whenever the all paths collapse to the whole-column path:

SELECT s FROM tbl WHERE struct_element(s, 'city') = 'x';
-- all access paths: [s], predicate access paths: [] (was [s.city])

BE then cannot tell which sub-column the predicate reads, so it loses the eager/lazy split and reads the column as one unit. On branch-4.2 this shows up in test_iceberg_variant_read, where the lazy-read check sees FilteredRowsByLazyRead = 0 for

SELECT CAST(v AS STRING) FROM variant_page_pruning WHERE CAST(v['n'] AS INT) > 3000

Only file scans hit this: on OLAP tables a variant sub-path predicate gets its own sub-column slot, so its all paths already contain the predicate path. That is why the existing unit tests did not catch it, and why the new test uses a struct column, which keeps one slot for the whole column.

This PR keeps the NULL/OFFSET cleanup, since BE switches the whole iterator to NULL_MAP_ONLY/OFFSET_ONLY once such a path shows up and skips the children. Every other predicate path is kept, and added to the all paths when no wider path covers it — the behaviour of master's addPredicatePathsToFinalAllAccessPaths.

Release note

Fix nested/variant sub-field predicates losing lazy materialization on branch-4.2 when the whole column is read.

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. Sub-field predicates keep their access paths, restoring predicate-first reads.

• 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

[hubgeter]

run buildall

[hello-stephen]

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1. What problem was fixed (it's best to include specific error reporting information). How it was fixed.
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[hello-stephen]

FE UT Coverage Report

Increment line coverage 60.00% (9/15) 🎉
Increment coverage report
Complete coverage report

[hubgeter]

/review