Support pl.Expr.dt.date

[mroeschke]

Description

xref #23151

https://docs.pola.rs/api/python/dev/reference/expressions/api/polars.Expr.dt.date.html

Checklist

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

[coderabbitai]

📝 Walkthrough

Walkthrough

This PR adds GPU support for dt.date() in TemporalFunction by accepting Date as a valid temporal op and evaluating it through a cast to the date storage type. It also adds parameterized test coverage across multiple datetime units.

Changes

Date Temporal Function Support

Layer / File(s)	Summary
Date op validation and evaluation python/cudf_polars/cudf_polars/dsl/expressions/datetime.py	TemporalFunction.Name.Date is added to _valid_ops, and do_evaluate gains a branch casting the input column to self.dtype.plc_type to drop the sub-day component.
Date function test coverage python/cudf_polars/tests/expressions/test_datetime_basic.py	New parameterized test_datetime_date test verifies .dt.date() GPU results across ms/us/ns datetime dtypes, including a pre-epoch datetime and a None value.

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

Possibly related PRs

• rapidsai/cudf#18443: Both PRs extend TemporalFunction in the same file by adding new ops to _valid_ops and new do_evaluate branches with matching test additions.
• rapidsai/cudf#22754: Both PRs modify TemporalFunction evaluation control flow for a temporal transform and add corresponding coverage in the datetime expression tests.

Suggested reviewers: vyasr, bdice

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
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.
Title check	✅ Passed	The title clearly and accurately summarizes the main change: adding support for pl.Expr.dt.date.
Description check	✅ Passed	The description is related to the change and links the relevant issue and API docs.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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[vyasr]

/merge