Add Ordering.as_strict helper

[rjzamora]

Adds a Python-facing Ordering.as_strict() helper so streaming code can reuse existing ordering boundaries while marking them strict.

Also updates ordered groupby metadata handling to use Ordering.with_keys() and Ordering.as_strict() instead of reconstructing Ordering objects manually.

Suggested by @wence- in #23306.

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• python/cudf_polars/cudf_polars/streaming/actor_graph/groupby.py
• python/cudf_streaming/cudf_streaming/tests/test_channel_metadata.py

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📝 Walkthrough

Summary by CodeRabbit

• New Features

  • Added Ordering.as_strict() to create equivalent orderings with strict boundaries.
  • Added support for constructing orderings from shared table data.

• Improvements

  • Improved ordering key remapping while preserving key-selection limits.
  • Updated streaming group-by processing for more consistent ordering behavior.

• Tests

  • Expanded coverage for strict ordering with populated, empty, and null boundaries, including alignment with the original ordering.

Walkthrough

The change adds Ordering.as_strict(), supports shared table-chunk construction, updates groupby ordering remapping, and replaces strict-ordering utility calls with the new method.

Changes

Ordering strictness integration

Layer / File(s)	Summary
Ordering API and validation python/cudf_streaming/cudf_streaming/channel_metadata.*, python/cudf_streaming/cudf_streaming/tests/test_channel_metadata.py	Ordering.as_strict() returns an equivalent ordering with strict boundaries. The declarations include the new method and shared table-chunk constructor. Tests cover populated, empty, and null boundaries.
Groupby ordering integration python/cudf_polars/cudf_polars/streaming/actor_graph/groupby.py	Groupby remaps keys with Ordering.with_keys() and uses partial_input_ordering.as_strict(). Runtime-only imports were removed. Chunkwise evaluation receives input metadata.

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

Merge Risk: ⚪ Minimal · up to 8ac99

This PR adds an ordering helper and updates related metadata handling without any supplied evidence of an actionable merge-blocking risk; it is merge-ready after normal checks and review.

Suggested reviewers: vyasr

🚥 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%. Docstring coverage is scoped to functions touched by this diff. Analyzed 14 functions across 3 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: adding the Python-facing Ordering.as_strict helper.
Description check	✅ Passed	The description accurately explains the new helper and the related ordered groupby metadata updates.
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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[coderabbitai]

Actionable comments posted: 1

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Inline comments:
In `@python/cudf_streaming/cudf_streaming/tests/test_channel_metadata.py`:
- Around line 234-242: Extend test_ordering_as_strict with cases using empty and
all-null boundary tables, including nullable mixed INT64 and STRING columns, and
assert that Ordering.as_strict() preserves valid keys, boundary counts,
strict-boundary metadata, and alignment. Keep the existing non-null case and
cover the required empty and all-null inputs without changing production code.

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Configuration used: Path: .coderabbit.yaml

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📥 Commits

Reviewing files that changed from the base of the PR and between 26eca08 and bf53cd8.

📒 Files selected for processing (5)

• python/cudf_polars/cudf_polars/streaming/actor_graph/groupby.py
• python/cudf_streaming/cudf_streaming/channel_metadata.pxd
• python/cudf_streaming/cudf_streaming/channel_metadata.pyi
• python/cudf_streaming/cudf_streaming/channel_metadata.pyx
• python/cudf_streaming/cudf_streaming/tests/test_channel_metadata.py

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[rjzamora]

/merge