the shared CatalogStackBase validator and PresetCatalog validator
checked parsed.netloc to enforce 'a valid URL with a host'. but netloc
is truthy for host-less URLs like https://:8080 or https://user@, so
those slipped through even though they have no host - contradicting the
error message. the workflow, step, and bundler validators already check
parsed.hostname (which is None in those cases); this aligns the two
stragglers with that. add regression tests covering port-only and
userinfo-only URLs.
