[Bug]: HNSW directories do not get deleted when the collection is deleted

### What happened?

When deleting collections, the HNSW directories do not get deleted along with them. This results in an ever increasing disk space usage. According to @tazarov, "looking at the rust implementation it seems that deleting the HNSW index dir is missing"

### Versions

Chroma 1.0.15+

### Relevant log output

```shell

```

Start by tracing the Rust collection-deletion path and how HNSW index directories are created and named. Confirm that deleting a collection leaves its HNSW directory behind, then verify that the directory is removed with the collection and disk usage no longer grows from retained indexes.

Written by the indexing model from the issue text.