refactor(s3_vectors): embed search queries through the shared vector store executor

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.