GH-51639: [C++][Python] Fix invalid result when casting struct with null-typed child to its own type

[kartik00052]

Rationale for this change

Casting a struct containing a null-typed field to its own type could produce an invalid array when the struct child length differed from the enclosing execution batch length, such as a list containing multiple structs in one row or a sliced struct array. The null child was created using the parent execution length instead of the input child span length.

What changes are included in this PR?

• Use the input ArraySpan length when creating null-filled cast results.
• Add C++ regression coverage for sliced structs and nested list/struct cases.
• Add PyArrow coverage for array, chunked array, compute, record batch, and Parquet casts.

Are these changes tested?

• C++ scalar cast regression tests added.
• PyArrow compute and Parquet regression tests added.
• Python syntax, C++ diagnostics, and git diff whitespace checks pass.
• Native C++ and PyArrow test execution is pending a compiler-enabled build environment; the current environment has no C/C++ compiler and no built local pyarrow extension.

Are there any user-facing changes?

Yes. Struct casts involving null-typed children now produce valid arrays instead of silently returning invalid arrays or raising during validation.

Critical Fix

This fixes incorrect or invalid data structures produced by a cast operation.

Was AI used for this PR?

• Human
• AI

The implementation and test changes were reviewed against the existing Arrow cast kernels, nested-array length handling, and repository conventions.

• GitHub Issue: [Python] Casting a struct with a null-typed field to its own type gives an invalid array #51639

[github-actions]

⚠️ GitHub issue #51639 has been automatically assigned in GitHub to PR creator.