Clean up range search

[magdalendobson]

While working on #1228, I noticed that there were some issues with our current implementation of range search and its testing.

The main issue with testing is that there were no tests ensuring the max_results parameter was respected. I have added two tests that ensure this now.

The main code had several issues with how max_results was handled:

1. The max_results parameter was allowed to be less than the initial L_search. This is a conceptual issue because the user expects max_results to stop the search from continuing for too long, and the compute used in the initial search will always be controlled by initial_search_l.
2. A max_results check was not enforced before deciding to continue to the second round search. This meant that if the max results was reached via the initial search, it might not be respected.
3. The second round search was not terminated when max_results was reached, meaning it would continue to perform unnecessary work.

This PR fixes these issues by adding additional checks of max_results at the correct points in the code.

[Copilot]

Pull request overview

This PR tightens DiskANN’s range search behavior around the max_results / max_returned parameter by (a) validating it against the initial search list size and (b) enforcing the cap when deciding whether to enter, and while executing, the second-round range expansion. It also adds golden-style tests intended to ensure the cap is respected.

Changes:

• Add validation that max_returned >= starting_l, plus new RangeSearchError variant.
• Prevent entering the second-round range search when max_returned is already reached after the initial phase.
• Terminate second-round expansion once max_returned is reached; add two new generated-range-search test cases.

Reviewed changes

Copilot reviewed 2 out of 4 changed files in this pull request and generated 5 comments.

File	Description
diskann/src/graph/search/range_search.rs	Enforces max_returned constraints and early-stops/avoids second-round expansion when the cap is reached.
diskann/src/graph/test/cases/range_search.rs	Adds two range-search regression tests intended to validate max_results behavior and second-round triggering.
diskann/test/generated/graph/test/cases/range_search/max_results_respected_means_no_second_round.json	New golden output for the “no second round” max-results test.
diskann/test/generated/graph/test/cases/range_search/max_results_respected_and_second_round_triggered.json	New golden output for the “second round triggered” max-results test.

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[codecov-commenter]

Codecov Report

❌ Patch coverage is 94.05941% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.16%. Comparing base (d523dce) to head (112abe8).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
diskann/src/graph/test/cases/range_search.rs	93.33%	6 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##             main    #1253    +/-   ##
========================================
  Coverage   90.15%   90.16%            
========================================
  Files         509      509            
  Lines       97079    97191   +112     
========================================
+ Hits        87521    87628   +107     
- Misses       9558     9563     +5     

Flag	Coverage Δ
miri	90.16% <94.05%> (+<0.01%)	⬆️
unittests	89.83% <94.05%> (+<0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
diskann/src/graph/search/range_search.rs	97.82% <100.00%> (+0.06%)	⬆️
diskann/src/graph/test/cases/range_search.rs	95.83% <93.33%> (-1.30%)	⬇️

... and 8 files with indirect coverage changes

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