Informative warnings for CAGRA construction with low intermediate degree

[landrumb]

Resolves #2033 with more informative warnings about the intermediate graph degree parameter.

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[coderabbitai]

📝 Walkthrough

Walkthrough

This PR improves diagnostic messaging for CAGRA graph building by adding a pre-build warning when intermediate graph degree is below recommended levels and enhancing the GPU pruning failure message with specific guidance and calculated recommendations for the intermediate degree parameter.

Changes

CAGRA Graph Degree Diagnostics

Layer / File(s)	Summary
Intermediate degree validation and diagnostic messages cpp/src/neighbors/detail/cagra/cagra_build.cuh, cpp/src/neighbors/detail/cagra/graph_core.cuh	check_graph_degree adds a warning when intermediate_degree falls below the recommended 1.5 * graph_degree, noting disconnection risk. prune_graph_gpu assertion failure now computes and reports the recommended intermediate degree and instructs users about degree requirements and potential invalid neighbor causes.

Estimated code review effort

🎯 1 (Trivial) | ⏱️ ~3 minutes

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
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.
Description check	✅ Passed	The pull request description clearly describes the purpose of the changes: adding informative warnings about the intermediate graph degree parameter to resolve issue #2033.
Title check	✅ Passed	The title accurately describes the main change: adding informative warnings for CAGRA construction when intermediate degree is low, which matches the modifications in both files.

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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[divyegala]

Could you make the PR title more specific and descriptive please? As we use squash commits, the PR title acts as the commit message after the PR is merged.

[cjnolet]

/ok to test 41ac9af