Handle int64 strings offsets in cudf::byte_cast API

[davidwendt]

Description

Fixes logic in the cudf::byte_cast API which converts a strings column into a list column and properly handles int64 offsets by casting them to INT32 if possible.

Reference #23685 (review)

Checklist

• I am familiar with the Contributing Guidelines.
• New or existing tests cover these changes.
• The documentation is up to date with these changes.

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[davidwendt]

@coderabbitai full review

[davidwendt]

/ok to test

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📥 Commits

Reviewing files that changed from the base of the PR and between 70bf0bc and 1a3593a.

📒 Files selected for processing (1)

• cpp/src/reshape/byte_cast.cu

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📝 Walkthrough

Summary by CodeRabbit

• Bug Fixes
  • Improved string byte-casting reliability by ensuring offsets use the correct 32-bit integer format.
  • Added compatibility handling for inputs with different offset types.

Walkthrough

The string byte-casting path now normalizes the offsets column to INT32 before constructing the lists column. The change also adds the required unary utility include and updates the copyright attribution.

Changes

String byte-casting

Layer / File(s)	Summary
Normalize offsets before list construction cpp/src/reshape/byte_cast.cu	The byte-casting path extracts the offsets child and casts it to INT32 when needed before calling make_lists_column. The file adds the unary utility header and updates the SPDX copyright attribution.

Estimated code review effort: 2 (Simple) | ~10 minutes

Merge Risk: ⚪ Minimal · up to 1a359

This localized fix is merge-ready after normal checks and review; no actionable merge-blocking risk remains.

Suggested reviewers: abigalekim

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly summarizes the main change: handling int64 string offsets in the cudf::byte_cast API.
Description check	✅ Passed	The description directly explains the int64 offset handling change and its conversion to INT32 when possible.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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