Fix cagra::optimize modifying the state of raft::resources

[achirkin]

The new, improved version of CAGRA optimize() function makes use of raft CUDA stream pool resource.
Setting the resource (raft::resource::set_cuda_stream_pool) affects the state of raft::resources and may lead to unintended consequences for a user.
Using non-const raft::resources will be enforced at compile time in rapidsai/raft#3005 , so this PR fixes the use case ahead of the upstream change by copying the resources handle.

[coderabbitai]

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ℹ️ Recent review info

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📥 Commits

Reviewing files that changed from the base of the PR and between 972de08 and 5b9a38e.

📒 Files selected for processing (1)

• cpp/src/neighbors/detail/cagra/graph_core.cuh

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📝 Walkthrough

Summary by CodeRabbit

• Refactor
  • Updated internal resource handling in graph optimization logic to prevent unintended state mutations during processing.

Walkthrough

The optimize() function signature is changed to accept resources as a const reference parameter (res_const), and the function now immediately constructs a mutable local copy (res) for internal operations. Comments document the rationale: stream-pool modification requires non-const resources, and this pattern isolates such modifications from affecting the caller's input.

Changes

Resource parameter isolation

Layer / File(s)	Summary
Resource parameter const-correctness with local copy cpp/src/neighbors/detail/cagra/graph_core.cuh	The optimize() function parameter is renamed to res_const and the function constructs a local mutable copy for stream-pool setup, with clarifying comments explaining the non-const requirement and isolation intent.

Estimated code review effort

🎯 1 (Trivial) | ⏱️ ~3 minutes

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title directly and clearly summarizes the main change: fixing cagra::optimize to avoid modifying raft::resources state.
Description check	✅ Passed	The description explains the problem (state modification of raft::resources), the solution (copying the resources handle), and the motivation (upstream RAFT changes requiring const resources).
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.

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch achirkin-patch-7

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[mfoerste4]

Thanks @achirkin for fixing this!

[tfeher]

LGTM.

[cjnolet]

Please add a GitHub issue for this and link it in the code so we don't lose sight of this.

[achirkin]

rapidsai/raft#3022

[achirkin]

/merge