docs: improve vector store config documentation and fix test isolation

tests/unit/router/test_embedding_precedence.py

@@ -5,9 +5,13 @@
 # the root directory of this source tree.
 
 
+import pytest
+
 from llama_stack.apis.models import ModelType
 from llama_stack.core.routers.vector_io import VectorIORouter
 
+pytestmark = pytest.mark.asyncio
+
 
 class _DummyModel:
     def __init__(self, identifier: str, dim: int):