The API passes provider.uid (a bare hostname) to test_connection(),
which misclassified it as a Docker Hub image reference and failed auth
against Docker Hub instead of the target registry.
Add _is_registry_url() to detect bare hostnames (dots in host, no
slash) and _test_registry_connection() to test connectivity via the
OCI catalog endpoint. Also fix the UI placeholder to show a valid
example without https://.