GH-50515: [C++][Compute] Respect parent validity bitmap when casting nested structs with non-nullable fields

[aaron-seq]

Rationale for this change

When casting a nested struct whose inner field changes from nullable to
non-nullable, CastStruct::Exec currently calls
in_values->GetNullCount() > 0 without considering the parent struct's
validity bitmap. This causes a spurious ArrowInvalid error when the
child's physical nulls are fully masked by null parent entries.

Reported in #50515.

What changes are included in this PR?

In scalar_cast_nested.cc, replace the unconditional rejection with a
three-way check:

1. Parent has no bitmap → child nulls are unmasked; reject.
2. Child has no bitmap → element-wise check against parent validity.
3. Both bitmaps present → use BinaryBitBlockCounter::NextAndNotWord()
   to detect parent-valid-AND-child-null positions without heap allocation.

Are these changes tested?

Five new C++ test cases in scalar_cast_test.cc:

• StructNestedNullabilityAbsentParent — true violation, must reject
• StructNestedNullabilityMasked — masked null, must succeed
• StructNestedNullabilitySliced — offset correctness for both outcomes
• StructNestedNullabilityAbsentChild — no nulls, must succeed
• StructNestedNullabilityDeep — 3-level nesting with masked null

One new Python test in test_compute.py:

• test_cast_struct_nested_nullability — end-to-end PyArrow validation
  covering success, rejection, and sliced array cases

Are there any user-facing changes?

Struct casts that were previously rejected with
ArrowInvalid: field '...' has nulls will now succeed when the nulls
are masked by parent-level nulls. This is a correctness fix, not a
behavior change — the previous behavior was incorrect per the Arrow
columnar format specification.

• GitHub Issue: [C++][Python] Casting nested structs fails when inner struct are non-nullable, but the field for the inner struct is nullable #50515

[github-actions]

⚠️ GitHub issue #50515 has been automatically assigned in GitHub to PR creator.