Migrate stream APIs from rmm::cuda_stream_view to cuda::stream_ref

[bdice]

Summary

Track the coordinated migration of stream APIs and call sites from rmm::cuda_stream_view to CCCL's cuda::stream_ref. This propagates cuda::stream_ref through RMM containers and memory resources, RAFT resource and handle APIs, downstream C++ interfaces, Python/Cython bindings, benchmarks, tests, and documentation.

This migrates affected cuVS API signatures and internal call sites while extracting raw stream handles only where CUDA, generated/JIT, or legacy APIs require them.

Depends on rapidsai/rmm#2372 and NVIDIA/raft#3129.

Tracked in rapidsai/build-planning#318.

Migrations

• Pass cuda::stream_ref through stream pools, resource accessors, conditionals, and downstream APIs without converting to rmm::cuda_stream_view
• Use cuda::stream_ref constructions for default/legacy/per-thread streams
  • rmm::cuda_stream_default ➡️ cuda::stream_ref{cudaStream_t{cudaStreamDefault}}
  • rmm::cuda_stream_legacy ➡️ cuda::stream_ref{cudaStreamLegacy}
  • rmm::cuda_stream_per_thread ➡️ cuda::stream_ref{cudaStreamPerThread}
• Use .get() when calling an API that requires a raw cudaStream_t, including CUDA runtime, library, CUB, and legacy API boundaries (previously rmm::cuda_stream_view used value())
• Use .sync() when synchronizing a cuda::stream_ref (previously rmm::cuda_stream_view used synchronize())
• Update Cython declarations and call sites to pass stream references directly where supported

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[tarang-jain]

@bdice can you review #2526 to ensure I am not contradicting what you are doing here (a cmake review is also required there)

[bdice]

Rebased onto preparation PR #2557 and validated against the rebased RMM/RAFT stack. Local build-cuvs -j0 completed successfully, and the focused IVF-PQ test passed. CI is rerunning at 35c0411f.