Fix group-key columns in SeriesGroupBy list-agg and any/all with as_index=False

[galipremsagar]

Description

Fixes two bugs in SeriesGroupBy aggregations with as_index=False, where the group-key columns are materialized as output columns. Both only surfaced with categorical group keys, which is why they showed up in the cudf.pandas tests/groupby/test_categorical.py::test_agg_list cases.

1. SeriesGroupBy.agg([func]) blanked the key-column name. A list aggregation produces a column MultiIndex; with as_index=False the key is inserted as a (key, "") tuple by reset_index. The result columns were collapsed with a blanket droplevel(0), which keeps the empty padding level and so renamed the key column to "" (e.g. ["a1", "var"] became ["", "var"]). It now keeps the key name for key columns and the aggregation name for value columns.

2. any/all corrupted the categorical key column. _bool_reduce implements any/all as max/min on bool-coerced columns, then casts the results back to bool. With as_index=False that loop also ran over the key columns inserted into the result, turning e.g. a categorical key [0, 1] into [False, True]. It now skips the group-key columns when coercing to bool.

This un-xfails the previously-failing tests/groupby/test_categorical.py::test_agg_list[...] cases in the cudf.pandas pandas test suite (and incidentally fixes test_category_order_reducer). cuDF's own groupby test-suite shows no regressions.

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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• python/cudf/cudf/pandas/scripts/pandas-testing-plugin.py

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📝 Walkthrough

Summary by CodeRabbit

• Bug Fixes

  • Fixed groupby boolean reductions that could incorrectly cast group-key columns to boolean when keys were included in the result.
  • Improved multi-level column aggregation so grouping-key names are preserved correctly in aggregated results.

• Tests

  • Removed an expected-failure entry for a categorical groupby aggregation test, reflecting updated test behavior.

Walkthrough

Prevents boolean coercion of grouping-key columns in DataFrame aggregation results and corrects MultiIndex-to-Index flattening for SeriesGroupBy. Updates pandas-testing expected-failure mappings to reflect the behavioral changes.

Changes

GroupBy Aggregation Fixes

Layer / File(s)	Summary
Boolean reduction and MultiIndex column handling in aggregation python/cudf/cudf/core/groupby/groupby.py	_bool_reduce now skips coercing columns matching group-key names to boolean when result is a DataFrame with as_index=False. SeriesGroupBy.agg rebuilds column index by preserving group-key names for padding entries where the second level is empty, otherwise uses the second-level label.
Test expectation updates for groupby aggregation python/cudf/cudf/pandas/scripts/pandas-testing-plugin.py	Removes expected-failure block for tests/groupby/test_categorical.py::test_agg_list cases and updates nearby test_apply.py groupby test expectation entries to reflect aggregation logic fixes.

Estimated code review effort

🎯 3 (Moderate) | ⏱️ ~20 minutes

Suggested reviewers

• bdice
• brandon-b-miller

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 50.00% 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 pull request title accurately describes the main fixes: addressing group-key column handling in SeriesGroupBy list-agg and any/all operations with as_index=False.
Description check	✅ Passed	The pull request description provides clear explanations of both bugs fixed, their root causes, and the solutions implemented, with direct connection to the changeset.
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.

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[galipremsagar]

/merge