fix(image): route bare registry hostnames to OCI catalog in test_connection

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://.

ui/components/providers/workflow/forms/connect-account-form.tsx

@@ -60,7 +60,7 @@ const getProviderFieldDetails = (providerType?: ProviderType) => {
     case "image":
       return {
         label: "Registry URL",
-        placeholder: "e.g. https://registry.example.com",
+        placeholder: "e.g. 123456789012.dkr.ecr.us-east-1.amazonaws.com",
       };
     case "oraclecloud":
       return {