Skip another test that fails due to a polars bug

[wence-]

Description

In #23969 we skipped some tests in Polars 1.39, since it has bugs. However, we didn't catch them all. Address that.

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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ℹ️ Recent review info

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📥 Commits

Reviewing files that changed from the base of the PR and between 953eff5 and 914969d.

📒 Files selected for processing (1)

• python/cudf_polars/tests/expressions/test_agg.py

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📝 Summary

Summary by CodeRabbit

• Tests
  • Updated aggregation test coverage to skip incompatible Polars versions affected by a literal-handling issue.

Walkthrough

The groupby scalar max_by/min_by test now skips Polars versions before 1.39 and documents the Polars 1.38 literal-handling bug.

Changes

Polars test compatibility

Layer / File(s)	Summary
Aggregation test version gate python/cudf_polars/tests/expressions/test_agg.py	The test now excludes Polars versions before 1.39 and documents the Polars 1.38 literal-handling bug.

Estimated code review effort: 1 (Trivial) | ~2 minutes

Merge Risk: ⚪ Minimal · up to 91496

This change skips the affected aggregation test on unsupported Polars versions while retaining coverage on Polars 1.39 and later. It is ready to merge.

Suggested reviewers: mroeschke, matt711

🚥 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%. Docstring coverage is scoped to functions touched by this diff. Analyzed 1 functions across 1 files.	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 identifies the main change: skipping an additional test because of a Polars bug.
Description check	✅ Passed	The description directly explains that the pull request extends existing test exclusions for Polars bugs.
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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✨ Finishing Touches 💡 1

🛠️ Fix failing CI checks 💡

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests

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[wence-]

/merge