POST /v1/vector_stores always minted a random UUID for the new store's id,
ignoring the request entirely. litellm-config.yaml's vector_store_registry
addresses this store by a fixed id (memory-and-notes), which could never
match a real row as a result — every search/write 404'd.
Added an optional id field to VectorStoreCreateRequest; create_vector_store
uses it when given, falls back to a random UUID otherwise (unchanged
behavior for callers that don't care).