GH-14736: [C++] Propagate struct validity into union children

[Hei-MaoM]

Rationale for this change

StructArray::GetFlattenedField currently combines a struct's validity
bitmap with a child by assigning it to buffer 0. Union arrays do not have
top-level validity buffers, so flattening a union field aborts even when
the parent bitmap is all-valid. When the parent contains nulls, that
validity must instead be represented by the union's child arrays.

What changes are included in this PR?

• Preserve the union's null top-level validity buffer and its type codes.
• Apply parent validity to every aligned child of a sparse union.
• Rebuild dense-union child segments in logical order so parent-null slots
  are null without invalidating valid slots that share the same original
  child offset.
• Preserve non-decreasing dense-union offsets and handle sliced arrays.
• Add sparse and dense regression tests.

Are these changes tested?

• cmake --build /build/cpp --target arrow-array-test -j 4
• /build/cpp/debug/arrow-array-test --gtest_filter=StructArray.FlattenSparseUnion:StructArray.FlattenDenseUnionWithSharedOffsets --gtest_brief=1
• /build/cpp/debug/arrow-array-test --gtest_brief=1
  • 1062 passed
  • 1 skipped because the local build does not enable the required threading configuration
• clang-format 18.1.8
• cpplint 1.6.1
• git diff --check

Are there any user-facing changes?

Yes. StructArray::GetFlattenedField and callers such as struct_field
and Flatten no longer crash when selecting a union field from a nullable
struct. Parent nulls are represented in the appropriate union children.
There are no public API changes.

This PR contains a "Critical Fix". It fixes a crash on valid input.

Closes #14736.

AI assistance disclosure

I used OpenAI Codex to help investigate the issue, draft parts of the
implementation and tests, and run verification. I reviewed the final diff
and am responsible for understanding, debugging, and maintaining the change.

• GitHub Issue: [C++] "struct_field" kernel crashing when selecting a Union field and parent StructArray has a validity bitmap #14736