Implement attaching files to vector stores

This adds the ability to attach files to vector
stores (client.vector_stores.files.create) for the OpenAI Vector
Stores Files API.

The initial implementation is only for Faiss, and tested via the
existing test_responses.py::test_response_non_streaming_file_search.

Signed-off-by: Ben Browning <bbrownin@redhat.com>

llama_stack/distribution/routers/vector_io.py

@@ -19,6 +19,7 @@ from llama_stack.apis.vector_io import (
     VectorStoreObject,
     VectorStoreSearchResponsePage,
 )
+from llama_stack.apis.vector_io.vector_io import VectorStoreChunkingStrategy, VectorStoreFileObject
 from llama_stack.log import get_logger
 from llama_stack.providers.datatypes import RoutingTable
 
@@ -254,3 +255,20 @@ class VectorIORouter(VectorIO):
             ranking_options=ranking_options,
             rewrite_query=rewrite_query,
         )
+
+    async def openai_attach_file_to_vector_store(
+        self,
+        vector_store_id: str,
+        file_id: str,
+        attributes: dict[str, Any] | None = None,
+        chunking_strategy: VectorStoreChunkingStrategy | None = None,
+    ) -> VectorStoreFileObject:
+        logger.debug(f"VectorIORouter.openai_attach_file_to_vector_store: {vector_store_id}, {file_id}")
+        # Route based on vector store ID
+        provider = self.routing_table.get_provider_impl(vector_store_id)
+        return await provider.openai_attach_file_to_vector_store(
+            vector_store_id=vector_store_id,
+            file_id=file_id,
+            attributes=attributes,
+            chunking_strategy=chunking_strategy,
+        )