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/remote/vector_io/milvus/milvus.py

@@ -247,6 +247,16 @@ class MilvusIndex(EmbeddingIndex):
     ) -> QueryChunksResponse:
         raise NotImplementedError("Hybrid search is not supported in Milvus")
 
+    async def delete_chunk(self, chunk_id: str) -> None:
+        """Remove a chunk from the Milvus collection."""
+        try:
+            await asyncio.to_thread(
+                self.client.delete, collection_name=self.collection_name, filter=f'chunk_id == "{chunk_id}"'
+            )
+        except Exception as e:
+            logger.error(f"Error deleting chunk {chunk_id} from Milvus collection {self.collection_name}: {e}")
+            raise
+
 
 class MilvusVectorIOAdapter(OpenAIVectorStoreMixin, VectorIO, VectorDBsProtocolPrivate):
     def __init__(
@@ -369,3 +379,13 @@ class MilvusVectorIOAdapter(OpenAIVectorStoreMixin, VectorIO, VectorDBsProtocolP
                 )
 
         return await index.query_chunks(query, params)
+
+    async def delete_chunks(self, store_id: str, chunk_ids: list[str]) -> None:
+        """Delete a chunk from a milvus vector store."""
+        index = await self._get_and_cache_vector_db_index(store_id)
+        if not index:
+            raise ValueError(f"Vector DB {store_id} not found")
+
+        for chunk_id in chunk_ids:
+            # Use the index's delete_chunk method
+            await index.index.delete_chunk(chunk_id)