[AURON #2472] Fix nullable struct field null propagation

[lyne7-sc]

Which issue does this PR close?

Closes #2472

Rationale for this change

Auron does not propagate the parent struct's null bitmap when accessing a child field. As a result, a field of a null struct may incorrectly return a physical value such as 0 instead of null.

What changes are included in this PR?

• Combine the parent struct and child field null bitmaps in GetIndexedFieldExpr.
• Preserve the zero-copy path when the parent struct contains no nulls.

Are there any user-facing changes?

Bug fix only, accessing a field of a null struct now returns null, consistent with vanilla Spark.

How was this patch tested?

• Add Rust tests covering parent and child null combinations.
• Add a Spark Parquet E2E test for nullable struct field access.

Was this patch authored or co-authored using generative AI tooling?

• Yes
• No

If yes, include: Generated-by: gpt-5.6

ASF guidance: https://www.apache.org/legal/generative-tooling.html

[Copilot]

Pull request overview

Fixes incorrect null handling when extracting a child field from a nullable StructArray in the native engine: the child field now becomes null whenever the parent struct is null (matching vanilla Spark semantics), while keeping the zero-copy path when the parent struct has no nulls.

Changes:

• Combine parent struct and child field null bitmaps in GetIndexedFieldExpr using NullBuffer::union.
• Preserve zero-copy behavior by returning the child array directly when the parent struct has no nulls.
• Add coverage via a Rust unit test for parent/child null propagation and a Spark Parquet E2E query test.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
native-engine/datafusion-ext-exprs/src/get_indexed_field.rs	Propagates parent struct nulls to the selected child field and adds Rust unit tests for the behavior (and zero-copy path).
spark-extension-shims-spark/src/test/scala/org/apache/auron/AuronQuerySuite.scala	Adds an E2E Parquet test ensuring s.child is null when s is null.

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