chore: use Uuid type throughout index infrastructure instead of String

[wjones127]

Several index-related types store or pass UUIDs as String/&str despite IndexMetadata.uuid already being the native Uuid type. This forces unnecessary .to_string() conversions at every callsite.

Changes, in dependency order:

1. CreateIndexBuilder.index_uuid: Option<String> → Option<Uuid> (lance/src/index/create.rs). The builder currently accepts a String and immediately calls Uuid::parse_str internally — accepting Uuid directly removes this round-trip.

2. DatasetIndexInternalExt trait methods open_generic_index, open_scalar_index, open_vector_index — change uuid: &str → uuid: &Uuid (lance/src/index.rs). Every call site converts from IndexMetadata.uuid (already Uuid) via .to_string(), which would be eliminated.

3. IVFIndex.uuid: String → Uuid in both lance/src/index/vector/ivf.rs and lance/src/index/vector/ivf/v2.rs. Unblocked by Python code dump #2 above.

4. ANNIvfPartitionExec.index_uuids: Vec<String> → Vec<Uuid> (lance/src/io/exec/knn.rs). Currently built by mapping IndexMetadata.uuid.to_string(). Unblocked by Python code dump #2 above.

5. Cache key structs in lance/src/session/index_caches.rs that use uuid: &'a str — standardize to &'a Uuid to match the one key that already uses &'a Uuid.