Adopt CUDA stream compatibility accessors

[bdice]

Summary

This PR helps RAFT adopt cuda::stream_ref. It is a non-breaking subset of the changes in #3129 to ease migration.

Use the get() and sync() compatibility aliases added in RMM #2537. These spellings are shared by rmm::cuda_stream_view and cuda::stream_ref.

This preserves existing stream types and public APIs while extracting mechanical accessor updates from the broader stream migration. It is independently buildable without RMM #2372 and leaves the migration PR focused on actual type and signature changes.

This updates raw CUDA, library, kernel-launch, and legacy API boundaries throughout RAFT and uses the CUDA stream umbrella header where stream definitions are required. Existing rmm::cuda_stream_view return types remain unchanged; their migration stays in RAFT #3129.

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• cpp/bench/prims/util/fast_int_div.cu

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📝 Summary

Summary by CodeRabbit

• Bug Fixes

  • Improved CUDA stream interoperability across communication, linear algebra, matrix, random, sparse, spectral, and statistics operations.
  • Ensured asynchronous GPU operations consistently receive valid native stream handles.
  • Improved stream synchronization reliability across GPU operations and benchmarks.

• Chores

  • Replaced deprecated CUDA stream headers with the current stream interface.
  • Updated validation, test, and benchmark coverage without changing public APIs or numerical behavior.

Walkthrough

Changes

CUDA stream handle migration

Layer / File(s)	Summary
Production stream handle conversion cpp/include/raft/**, cpp/src/**	Production code now passes raw cudaStream_t values from stream wrappers through .get(). Deprecated cuda/stream_ref includes and .value() access are replaced.
Test and benchmark stream conversion cpp/tests/**, cpp/bench/**	Tests and benchmarks now pass raw CUDA stream handles to CUDA, RAFT, RMM, and library APIs. Updated synchronization calls use sync().

Estimated code review effort: 3 (Moderate) | ~20 minutes

Merge Risk:⚪ Minimal · up to e4058

This updates benchmark stream accessor calls without changing public APIs or stream ordering. No current merge-blocking risk is identified.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 26.73% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 101 functions across 66 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: adopting CUDA stream compatibility accessors.
Description check	✅ Passed	The description directly explains the stream accessor and synchronization updates, compatibility goals, preserved APIs, and migration scope.

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