This commit modifies HNSWlib to filter duplicate document IDs during KNN
search, ensuring only one embedding per unique document ID is returned.
Key changes include:

- Added `internal_id_to_doc_id_` vector to `HierarchicalNSW` to map internal
  IDs to document IDs, populated in `addPoint`.
- Introduced `getMetadata` method to retrieve document IDs.
- Extended `VisitedList` with `seen_doc_ids` set to track seen document IDs
  thread-locally, avoiding mutex contention.
- Updated `searchBaseLayerST` to skip candidates with already-seen document
  IDs using `vl->is_doc_seen(doc_id)`.
- Removed unused `visited_metadata_` and `visited_metadata_lock_` as filtering
  is now handled by `VisitedList`.
The duplicate filtering works as intended, though `knnQuery` may raise a
`RuntimeError` if `k` exceeds the number of unique document IDs due to
result array shape constraints. Tests for basic filtering, single ID, and
large datasets pass, while empty index and insufficient IDs cases require
further handling.

Files modified:
- hnswalg.h: Added duplicate filtering logic and mappings.
- visited_list_pool.h: Enhanced `VisitedList` for document ID tracking.