Fix RMM race conditions from deferred H2D copies in pylibcudf tests

[Matt711]

Description

Split out from #23517.

DeviceBuffer.to_device(bytes, stream) reads host memory asynchronously (rapidsai/rmm#2511). If the bytes object isn't kept alive past the copy, Python can free it before the GPU reads it.

The PR keeps those bytes alive through the copy by making an explicit reference. This fixes the immediate problem but it not the end of this problem. Let's use rapidsai/rmm#2521 to discuss a more general solution or at minimum document the current behavior.

I tested these changes in #23517 (which reliably reproduced the bug). And the failing tests fixed in this PR are now passing.

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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📥 Commits

Reviewing files that changed from the base of the PR and between 8c3955b and 2242eb3.

📒 Files selected for processing (3)

• python/pylibcudf/tests/io/test_experimental_hybrid_scan.py
• python/pylibcudf/tests/io/test_parquet.py
• python/pylibcudf/tests/test_column_from_device.py

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📝 Walkthrough

Summary by CodeRabbit

• Bug Fixes

  • Improved reliability when loading Parquet data and converting columns from device buffers.
  • Prevented intermittent issues caused by asynchronous transfers using temporary source data.

• Documentation

  • Clarified that source data must remain available until device transfers are complete.

• Tests

  • Strengthened coverage for hybrid scans, Parquet device-buffer handling, and column conversion workflows.

Walkthrough

The tests now materialize host byte sources, retain them during asynchronous host-to-device copies, and synchronize streams before dependent device-buffer use.

Changes

Asynchronous host-to-device copy lifetime

Layer / File(s)	Summary
Retain host bytes through device copies python/pylibcudf/tests/io/test_experimental_hybrid_scan.py, python/pylibcudf/tests/io/test_parquet.py, python/pylibcudf/tests/test_column_from_device.py	Tests keep source bytes in local variables during asynchronous copies. Parquet buffer use now follows immediate stream synchronization. Comments document the source-lifetime requirement.

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

Merge Risk: ⚪ Minimal · up to 2242e

This localized test-only change keeps host data alive during asynchronous device copies; no actionable merge-blocking risk remains beyond normal checks and review.

Suggested reviewers: bdice, vyasr, mhaseeb123

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 25.00% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 4 functions across 3 files.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
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 summarizes the main change: fixing RMM race conditions caused by deferred host-to-device copies in pylibcudf tests.
Description check	✅ Passed	The description directly explains the race condition, the explicit byte-reference fix, affected behavior, and test validation.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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[Matt711]

/merge