The Router executor only routed a query embedding when the vector store
carried extra embedding configuration, so a store registered with no
embedding model at all always went to the Router and 500'd on the
s3_vectors default text-embedding-3-small when no deployment served it.
Route on whether the Router serves the model, which is the rule the
executor had before, and keep the request metadata on the SDK fallback so
the embedding stays attributed either way.
S3 Vectors now subclasses BaseQueryEmbeddingVectorStoreConfig, so its query
embedding runs through the Router executor with the request metadata instead
of a private router lookup. embedding_model stays accepted as an alias of
litellm_embedding_model. The router kwarg is gone from the search handler and
every provider transform now that nothing but the executor fallback read it.
