feat: implement chunk deletion for vector stores

Add support for deleting individual chunks from vector stores

- Add abstract delete_chunk() method to EmbeddingIndex base class
- Implement chunk deletion for Faiss provider with index tracking
- Add chunk_ids list to maintain chunk order in Faiss index
- Integrate chunk deletion into OpenAI vector store file deletion flow
- Add placeholder implementations for SQLite and Milvus providers

Closes: #2477

Signed-off-by: Derek Higgins <derekh@redhat.com>

llama_stack/providers/remote/vector_io/milvus/milvus.py

@@ -369,3 +369,7 @@ class MilvusVectorIOAdapter(OpenAIVectorStoreMixin, VectorIO, VectorDBsProtocolP
                 )
 
         return await index.query_chunks(query, params)
+
+    async def delete_chunk(self, store_id: str, chunk_id: str) -> None:
+        """Delete a chunk from a milvus vector store."""
+        pass  # TODO