copy bftrees from the snapshot location to the save location

[backurs]

This is a small PR that makes sure that we copy on-disk bftree index files from the snapshot() location to the location as specified when saving the bf-tree based index.

[Copilot]

Pull request overview

This PR updates bf-tree index persistence so that when saving an on-disk bf-tree-based index, the generated snapshot files are copied from the bf-tree’s internal snapshot location to the save prefix location provided to save_with().

Changes:

• Change bf-tree provider snapshot() helpers to return the snapshot PathBuf.
• Update BfTreeProvider::save_with() to copy vector/neighbor/(quant) .bftree snapshot files to the target prefix paths when they differ.

Reviewed changes

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

File	Description
diskann-providers/src/model/graph/provider/async_/bf_tree/vector_provider.rs	Make snapshot() return the underlying bf-tree snapshot path (PathBuf).
diskann-providers/src/model/graph/provider/async_/bf_tree/quant_vector_provider.rs	Make snapshot() return the underlying bf-tree snapshot path (PathBuf).
diskann-providers/src/model/graph/provider/async_/bf_tree/neighbor_provider.rs	Make snapshot() return the underlying bf-tree snapshot path (PathBuf).
diskann-providers/src/model/graph/provider/async_/bf_tree/provider.rs	Copy .bftree snapshot outputs to the save prefix paths during save_with().

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.00%. Comparing base (7cd231a) to head (665937c).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #783      +/-   ##
==========================================
- Coverage   89.00%   89.00%   -0.01%     
==========================================
  Files         428      428              
  Lines       78417    78417              
==========================================
- Hits        69795    69793       -2     
- Misses       8622     8624       +2     

Flag	Coverage Δ
miri	89.00% <ø> (-0.01%)	⬇️
unittests	89.00% <ø> (-0.01%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.
see 1 file with indirect coverage changes

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.