GH-50524: [C++] Honor array offset in pairwise_diff

[Krishnanand-G]

Rationale for this change

ArraySpan::SetSlice replaces the offset. pairwise_diff copied the input span and then sliced from left_start/right_start, so a sliced array was read from the parent buffer. With [99, 1, 4, 9, 16] sliced to [1, 4, 9, 16], period=1 produced [-98, 3, 5, 7] instead of [null, 3, 5, 7].

What changes are included in this PR?

The kernel passes input.offset + left_start (and the same for the right side). The regression test uses a sliced int64 array for both period signs, on pairwise_diff and pairwise_diff_checked.

Are these changes tested?

Yes, TestPairwiseDiff.SlicedInput in vector_pairwise_test.cc.

Are there any user-facing changes?

pairwise_diff on a sliced array now diffs the sliced values. Callers who passed a slice and got parent-buffer values will see different (correct) output.

• GitHub Issue: [C++][Python] pairwise_diff ignores array offsets for sliced inputs #50524

[github-actions]

⚠️ GitHub issue #50524 has been automatically assigned in GitHub to PR creator.

[github-actions]

⚠️ GitHub issue #50524 has no components, please add labels for components.

[uros-b]

LGTM

[Krishnanand-G]

I checked the ARM64 macOS failures.

ARM64 macOS 14 C++: arrow-s3fs-test timed out at 300s (two retries). arrow-compute-vector-test passed (4.56s), including vector_pairwise_test.cc.

ARM64 macOS GLib & Ruby: the build died in vendored google-cloud-cpp (absl::visit marked deprecated, compiled with -Werror) while compiling filesystem/gcsfs.cc. vector_pairwise.cc compiled before that.

Same commit is green on ARM64 Ubuntu C++ and AMD64 macOS 15-intel C++.

#50524 still has no component labels. I don't have permission to add Component: C++ / Component: Python on the issue. A committer can add those if needed.

[Krishnanand-G]

This has @uros-b's LGTM and the two ARM64 macOS failures are unrelated (S3 test timeout + google-cloud-cpp Werror on deprecated absl::visit — both green on ARM64 Ubuntu and AMD64 macOS). Could a committer add the component labels to #50524 and merge this?

[pitrou]

Thanks for the fix @Krishnanand-G ! This looks good to me.