Add MultiInsertStrategy implementations for BfTreeProvider

[hildebrandmw]

These implementations were missed in #859 which broke some internal test code. No tests are added since the internal code that broke was largely serving as the tests, and we don't seem to have any current tests for BfTreeProvider anyways.

[Copilot]

Pull request overview

Note

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Adds missing MultiInsertStrategy implementations for BfTreeProvider to restore internal test/usage paths that broke after #859.

Changes:

• Introduce a pq::Overlay type alias for HybridMap working-set overlays.
• Implement MultiInsertStrategy for BfTreeProvider in both FullPrecision and Hybrid modes.

Reviewed changes

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

File	Description
diskann-providers/src/model/graph/provider/async_/distances.rs	Adds an overlay type alias used as the seed for multi-insert flows with PQ/hybrid distances.
diskann-providers/src/model/graph/provider/async_/bf_tree/provider.rs	Adds MultiInsertStrategy impls so batch insert paths can build the appropriate working set/overlay for BfTreeProvider.

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[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.31%. Comparing base (779977c) to head (fdb3cca).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #949      +/-   ##
==========================================
- Coverage   89.32%   89.31%   -0.01%     
==========================================
  Files         447      447              
  Lines       83891    83891              
==========================================
- Hits        74934    74931       -3     
- Misses       8957     8960       +3     

Flag	Coverage Δ
miri	89.31% <ø> (-0.01%)	⬇️
unittests	89.16% <ø> (-0.01%)	⬇️

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

Files with missing lines	Coverage Δ
...iders/src/model/graph/provider/async_/distances.rs	47.88% <ø> (ø)

... and 1 file with indirect coverage changes

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[harsha-simhadri]

are there tests already to cover this code path? if not, please add.

[hildebrandmw]

I called this out in the PR description: there are no meaningful tests for BfTreeProvider. The only tests were code in our internal Python package (which I broke with the multi-insert change and this is fixing 😢). Writing tests here would involve standing up the entire test ecosystem for the BfTreeProvider, which I really don't think is a good use of time at the moment. In any case, this particular change is 95% checked by the compiler.