feat(rust): add serialize/deserialize to brute force index

[jamie8johnson]

Add serialize/deserialize to the Rust brute force index

What

Adds serialize and deserialize to the Rust brute_force::Index, wrapping the
existing C entry points cuvsBruteForceSerialize / cuvsBruteForceDeserialize.
This brings the brute force binding to parity with the CAGRA binding, which already
exposes serialize/deserialize.

• Index::serialize(&self, res, filename) writes the index to disk.
• Index::deserialize(res, filename) -> Result<Index> loads an index from disk.

Both methods mirror the CAGRA implementation:

• A private path_to_cstring helper converts the filesystem path to a CString,
  returning Error::InvalidArgument (instead of panicking) for paths that are not
  valid UTF-8 or that contain an interior NUL byte. The path is validated before any
  FFI call is made.
• Every FFI call is wrapped in check_cuvs.
• deserialize constructs the Index handle first (via Index::new()), so that if
  the underlying cuvsBruteForceDeserialize call fails, the handle's Drop still
  runs and releases the C-side index allocation (RAII-safe error path).

The doc comments note that the serialization format may change between cuVS versions,
matching the wording in the C header.

Notes for reviewers

• No new bindings were generated. cuvsBruteForceSerialize and
  cuvsBruteForceDeserialize are already present in rust/cuvs-sys/src/bindings.rs
  (brute_force is pulled in through core/all.h), so this change is purely additive
  on the safe Rust wrapper side and touches no generated code.
• Test helper lifetime detail. The brute force Index keeps a non-owning device
  view of its dataset (_dataset). The serialize round-trip test deliberately keeps
  the host ndarray array in the same scope as the index for the duration of the
  test, because the device tensor's shape pointer borrows that array's dimension
  storage. Moving the host array while the index is alive would dangle that pointer
  (this is a property of the existing ManagedTensor view, not of these new methods).
• Conflicts with sibling in-flight Rust PRs (e.g. the IVF-SQ bindings PR feat(rust): add IVF-SQ bindings #2229
  and other Rust binding PRs): if conflicts arise, resolve by merging main into this
  branch rather than rebasing, per the project's no-rebase contribution guideline.

Testing

Two new unit tests were added alongside the existing test_l2, mirroring the CAGRA
serialize tests:

• test_brute_force_serialize_deserialize — builds an index, serializes it, asserts
  the output file exists and is non-empty, deserializes it back, and re-verifies that
  a self-neighbor search on the loaded index returns each query as its own nearest
  neighbor.
• test_brute_force_serialize_rejects_interior_nul — confirms that a path containing
  an interior NUL byte surfaces as Error::InvalidArgument rather than panicking.

All brute force tests pass (run single-threaded on a single GPU):

cargo test -p cuvs brute_force -- --test-threads=1
test brute_force::tests::test_brute_force_serialize_deserialize ... ok
test brute_force::tests::test_brute_force_serialize_rejects_interior_nul ... ok
test brute_force::tests::test_l2 ... ok
test result: ok. 3 passed; 0 failed; 0 ignored

cargo fmt and cargo clippy are clean for the changed file.

On path_to_cstring: intentionally mirrored byte-for-byte from cagra/index.rs (a third copy arrives with the in-flight IVF-SQ PR #2229). Happy to hoist it into a shared module here or as a follow-up once the serialize PRs settle — whichever you prefer.

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[coderabbitai]

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• rust/cuvs/src/brute_force.rs

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📝 Walkthrough

Summary by CodeRabbit

• New Features

  • Brute-force KNN indices can now be serialized to and deserialized from files.
  • Enhanced error handling for invalid file paths.

• Tests

  • Added comprehensive test coverage for index serialization, deserialization, and error handling scenarios.

Walkthrough

This PR adds file serialization support to the Rust brute-force KNN Index, introducing a safe path-to-CString helper, public serialize/deserialize methods that wrap cuVS C-API calls, and comprehensive tests for round-trip serialization and invalid path handling.

Changes

Index Serialization Support

Layer / File(s)	Summary
Serialization API and path helper rust/cuvs/src/brute_force.rs	Imports CString and Path; adds private path_to_cstring helper that rejects non-UTF-8 and interior-NUL paths with Error::InvalidArgument; adds public Index::serialize and Index::deserialize methods that route paths through the helper and call cuVS FFI, with deserialize allocating the Index handle before FFI to ensure RAII cleanup on error.
Serialization tests rust/cuvs/src/brute_force.rs	Introduces test helpers for dataset generation and search verification; adds round-trip serialize/deserialize test confirming top-1 self-neighbor behavior after file reload; adds test verifying interior-NUL filenames return Error::InvalidArgument.

🎯 2 (Simple) | ⏱️ ~12 minutes

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly and concisely describes the main change: adding serialize/deserialize functionality to the brute force index in Rust.
Description check	✅ Passed	The description is comprehensive and directly related to the changeset, explaining what was added, how it works, testing details, and implementation notes.
Docstring Coverage	✅ Passed	Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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[coderabbitai]

Actionable comments posted: 2

🤖 Prompt for all review comments with AI agents

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

Inline comments:
In `@rust/cuvs/src/brute_force.rs`:
- Around line 269-270: The test currently uses a fixed temp filename
("test_brute_force_index.bin") which can collide across parallel runs; replace
that with a unique temporary file using the tempfile crate (e.g., create a
tempfile::NamedTempFile and use its path for serialization), or generate a
unique filename (PID + UUID) in the temp dir; update the code that builds
filepath (currently the std::env::temp_dir().join(...) expression) to use the
NamedTempFile path (or unique name) and ensure the tempfile crate is added to
Cargo.toml if not already present so index.serialize(&res, &filepath) writes to
a non-colliding location.

In `@UPSTREAM_PR_BODY.md`:
- Around line 57-63: The fenced code block in UPSTREAM_PR_BODY.md is missing a
language identifier which triggers MD040; update the triple-backtick fence that
surrounds the cargo test output to include a language (e.g., change ``` to
```text) so the block is explicitly marked as plain text and the markdownlint
warning is resolved.

🪄 Autofix (Beta)

Fix all unresolved CodeRabbit comments on this PR:

• Push a commit to this branch (recommended)
• Create a new PR with the fixes

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• UPSTREAM_PR_BODY.md
• rust/cuvs/src/brute_force.rs