[diskann-providers] Remove the LRU dependency and make vfs and serde_json optional.

[hildebrandmw]

Remove the LRU crate as a dependency of diskann-providers since it is unused and triggering a dependabot alert. In addition, make the following dependencies optionsl:

• vfs - activated by the "virtual_storage" feature.
• serde_json - activated by the "bf_tree" feature.

Finally, cbindgen and csv are removed from the workspace level Cargo.toml since they are unused.

[Copilot]

Pull request overview

This PR streamlines the diskann-providers crate’s dependency graph by removing an unused lru dependency (addressing an alert) and by making vfs and serde_json opt-in via crate features, reducing default build surface area.

Changes:

• Removed the unused lru dependency from diskann-providers.
• Made serde_json optional and enabled only via the bf_tree feature.
• Made vfs optional and enabled only via the virtual_storage feature (while keeping it available for tests/benches via dev-dependencies).

Reviewed changes

Copilot reviewed 2 out of 3 changed files in this pull request and generated no comments.

File	Description
diskann-providers/Cargo.toml	Drops lru; makes vfs/serde_json optional and wires them to virtual_storage/bf_tree features; keeps vfs in dev-deps for tests/benches.
Cargo.toml	Removes unused workspace dependency entries (cbindgen, csv).
Cargo.lock	Updates lockfile to reflect removal of lru and related transitive dependency resolution changes.

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[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.01%. Comparing base (f3fcaae) to head (b43e96f).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #733   +/-   ##
=======================================
  Coverage   89.01%   89.01%           
=======================================
  Files         428      428           
  Lines       78294    78294           
=======================================
+ Hits        69692    69693    +1     
+ Misses       8602     8601    -1     

see 4 files with indirect coverage changes

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