Fix groupby.apply columns axis name for row-like UDF results

[galipremsagar]

Description

Fixes a cudf.pandas correctness bug in DataFrameGroupBy.apply: when the UDF returns a row-like Series (e.g. x.iloc[0]), the result's columns axis now inherits that Series' name to match pandas, instead of being left unnamed.
Re-enables the now-passing test_callable_result_dtype_frame cases in the pandas-test plugin.

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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📝 Walkthrough

Walkthrough

GroupBy.apply for row-like Series outputs now explicitly preserves the columns-axis name after transposing and concatenating results. The corresponding test entries are removed from the failure registry, indicating those tests now pass with the fix.

Changes

GroupBy apply columns metadata

Layer / File(s)	Summary
Preserve columns-axis name in groupby.apply python/cudf/cudf/core/groupby/groupby.py, python/cudf/cudf/pandas/scripts/pandas-testing-plugin.py	In _post_process_chunk_results, the columns-axis name is explicitly set from the first chunk's Series name after transpose/concat operations. A large block of test_callable_result_dtype_frame[apply-...] test entries is removed from the pandas testing plugin's failure registry, reflecting that these tests now pass.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~8 minutes

Suggested labels

bug, non-breaking

Suggested reviewers

• TomAugspurger
• mroeschke

🚥 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 and specifically describes the main change: fixing the columns axis name for row-like UDF results in groupby.apply.
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.
Description check	✅ Passed	The pull request description clearly explains the bug fix for cudf.pandas correctness in DataFrameGroupBy.apply and references the related code changes documented in the raw summary.

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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[galipremsagar]

/merge