feat: implement chunk deletion for vector stores (#2701)

Add support for deleting individual chunks from vector stores

- Add abstract remove_chunk() method to EmbeddingIndex base class
- Implement chunk deletion for Faiss provider, SQLite Vec, Milvus,
PGVector
- Placeholder implementations with NotImplementedError for
Chroma/Qdrant/Weaviate
- Integrate chunk deletion into OpenAI vector store file deletion flow
- removed xfail from
test_openai_vector_store_delete_file_removes_from_vector_store

Closes: #2477

---------

Signed-off-by: Derek Higgins <derekh@redhat.com>
Co-authored-by: Francisco Arceo <arceofrancisco@gmail.com>

llama_stack/providers/registry/vector_io.py

@@ -410,6 +410,7 @@ See [PGVector's documentation](https://github.com/pgvector/pgvector) for more de
 """,
             ),
             api_dependencies=[Api.inference],
+            optional_api_dependencies=[Api.files],
         ),
         remote_provider_spec(
             Api.vector_io,