Fix recall computation for beta-filtered search in benchmark

[magdalendobson]

It turns out that when we implemented the fix to recall computation for filtered search in #1069, the fix did not propagate to beta search. This happened because beta search used the same runner as regular knn search in search/knn.rs, so the wrong GroundTruthMode was passed in. This PR fixes that problem by including GroundTruthMode in SearchSteps so it can be passed in earlier in the stack.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 78.26087% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.03%. Comparing base (ee7dbaa) to head (8c5f07e).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
diskann-benchmark/src/index/search/knn.rs	83.33%	3 Missing ⚠️
diskann-benchmark/src/index/benchmarks.rs	60.00%	2 Missing ⚠️

❌ Your patch status has failed because the patch coverage (78.26%) is below the target coverage (90.00%). You can increase the patch coverage or adjust the target coverage.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1238      +/-   ##
==========================================
- Coverage   91.01%   90.03%   -0.99%     
==========================================
  Files         501      504       +3     
  Lines       95572    95983     +411     
==========================================
- Hits        86988    86415     -573     
- Misses       8584     9568     +984     

Flag	Coverage Δ
miri	90.03% <78.26%> (-0.99%)	⬇️
unittests	89.69% <78.26%> (-1.29%)	⬇️

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

Files with missing lines	Coverage Δ
diskann-benchmark/src/index/inmem/spherical.rs	100.00% <ø> (ø)
diskann-benchmark/src/index/benchmarks.rs	66.66% <60.00%> (ø)
diskann-benchmark/src/index/search/knn.rs	78.84% <83.33%> (+1.06%)	⬆️

... and 47 files with indirect coverage changes

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[Copilot]

Pull request overview

This PR fixes recall computation for beta-filtered (and other range-groundtruth) search benchmarks by ensuring the correct GroundTruthMode is threaded into the KNN runner earlier in the call stack, avoiding the previous hardcoded mode mismatch.

Changes:

• Add groundtruth_mode: GroundTruthMode to SearchSteps (defaulting to Fixed) and pass it through Knn::search_all.
• Update KNN runner implementations to accept a groundtruth_mode parameter instead of hardcoding Fixed/Flexible.
• Set GroundTruthMode::Flexible at beta-filtered / multihop / inline-filter benchmark call sites that load range groundtruth.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
diskann-benchmark/src/index/search/knn.rs	Threads GroundTruthMode via SearchSteps into the KNN search runner/aggregator.
diskann-benchmark/src/index/inmem/spherical.rs	Marks beta-filtered/multihop/inline-filter in-memory phases as GroundTruthMode::Flexible.
diskann-benchmark/src/index/benchmarks.rs	Marks beta-filtered/multihop/inline-filter benchmark phases as GroundTruthMode::Flexible.

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