branch-4.1: [fix](iceberg) Avoid BE abort on partition predicates

[suxiaogang223]

What problem does this PR solve?

Issue Number: DORIS-27947

Related PR: #66012, #66481

Problem Summary:

On branch-4.1, Iceberg identity partition columns are classified as partition keys and are therefore omitted from the Parquet file-column schema tree. A predicate on such a column still reaches the Parquet file-level predicate probe. StructNode::children_column_exists() assumed that every probed column had a schema-tree entry, so a partition predicate could trigger a DCHECK and abort the BE.

This change makes the existence probe return false when the schema tree has no entry for the column. Parquet then skips file-level predicate pushdown for that partition column while the existing partition-value fill and upper-level predicate evaluation remain in effect. The added unit test covers present, known-missing, and absent schema-tree children.

Release note

Fix a BE crash when querying Iceberg branch data with an identity partition predicate.

Validation

• Formatted the changed files with clang-format 16.0.6.
• git diff --check passed.
• BE compilation and unit tests were not run locally because of their cost.

Behavior changed

Yes. A column absent from the file schema tree is now treated as unavailable for file-level predicate pushdown instead of terminating the BE.

[hello-stephen]

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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

[hello-stephen]

BE UT Coverage Report

Increment line coverage 100.00% (2/2) 🎉

Increment coverage report
Complete coverage report

Category	Coverage
Function Coverage	58.92% (25251/42856)
Line Coverage	43.38% (256880/592188)
Region Coverage	39.39% (203394/516395)
Branch Coverage	40.89% (93773/229346)

[suxiaogang223]

run external

[hello-stephen]

BE Regression && UT Coverage Report

Increment line coverage 100.00% (2/2) 🎉

Increment coverage report
Complete coverage report

Category	Coverage
Function Coverage	73.89% (30861/41766)
Line Coverage	58.03% (341092/587738)
Region Coverage	54.87% (284042/517701)
Branch Coverage	55.76% (127743/229076)