Fix save_path silently ignored in bftree benchmarks

[Copilot]

save_path in graph-index-bftree and graph-index-bftree-spherical JSON configs had no effect: both BfTreeFullPrecision and BfTreeSpherical benchmark backends built the index but never persisted it.

Changes

• bftree/full_precision.rs — after run_build + checkpoint, call save_with on the provider when save_path is set
• bftree/spherical.rs — same, after single-insert build + checkpoint

Both follow the existing pattern in the non-bftree FullPrecision benchmark. BfTreeProvider already implements SaveWith<String> for both NoStore and QuantVectorProvider — it just wasn't being called.

// save the index if requested
if let Some(save_path) = input.build().save_path() {
    tokio::block_on(
        index
            .provider()
            .save_with(&FileStorageProvider, &save_path.to_string()),
    )?;
}

[JordanMaples]

I'll pick this up on a new branch. In my local testing of this, there were some problems with saving the quantized tree. It looks like by upgrading to the 0.5.x versions of bftree, this might be sorted.

[JordanMaples]

Closing this in favor of #1183