chore: Adding unit tests for OpenAI vector stores and migrating SQLite-vec registry to kvstore (#2665)

# What does this PR do?

This PR refactors and the VectorIO backend logic for `sqlite-vec` and
adds unit tests and fixtures to make it easy to test both `sqlite-vec`
and `milvus`.

Key changes:
- `sqlite-vec` migrated to `kvstore` registry
- added in-memory cache for sqlite-vec to be consistent with `milvus`
- default fixtures moved to `conftest.py` 
- removed redundant tests from sqlite`-vec`
- made `test_vector_io_openai_vector_stores.py` more easily extensible 


## Test Plan
Unit tests added testing inline providers.

---------

Signed-off-by: Francisco Javier Arceo <farceo@redhat.com>

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

@@ -61,6 +61,11 @@ class MilvusIndex(EmbeddingIndex):
         self.consistency_level = consistency_level
         self.kvstore = kvstore
 
+    async def initialize(self):
+        # MilvusIndex does not require explicit initialization
+        # TODO: could move collection creation into initialization but it is not really necessary
+        pass
+
     async def delete(self):
         if await asyncio.to_thread(self.client.has_collection, self.collection_name):
             await asyncio.to_thread(self.client.drop_collection, collection_name=self.collection_name)
@@ -199,6 +204,9 @@ class MilvusVectorIOAdapter(OpenAIVectorStoreMixin, VectorIO, VectorDBsProtocolP
         if vector_db_id in self.cache:
             return self.cache[vector_db_id]
 
+        if self.vector_db_store is None:
+            raise ValueError(f"Vector DB {vector_db_id} not found")
+
         vector_db = await self.vector_db_store.get_vector_db(vector_db_id)
         if not vector_db:
             raise ValueError(f"Vector DB {vector_db_id} not found")