GH-49110: [C++] Add bounds checking to DataType::field() to return nullptr for out-of-bounds access

[HyukjinKwon]

Rationale for this change

Previously, DataType::field(int i) directly accessed children_[i] without bounds checking, resulting in undefined behavior for out-of-bounds indices. This could crash or return garbage.

What changes are included in this PR?

Added bounds checking to DataType::field() to return nullptr for out-of-bounds access, matching the behavior of GetFieldByName().

Are these changes tested?

Yes, added TestStructType.FieldAccessOutOfBounds to verify that out-of-bounds indices return nullptr.

Are there any user-facing changes?

Yes. DataType::field(i) now safely returns nullptr for invalid indices instead of undefined behavior.

• GitHub Issue: [C++] Add bounds checking to DataType::field() to return nullptr for out-of-bounds access #49110

[github-actions]

⚠️ GitHub issue #49110 has been automatically assigned in GitHub to PR creator.