Validate whether registry mirror is public in preflight validations

If the registry mirror provided is not set to public, we throw an error saying that it is unauthorized. For the create flow, it would be better if we throw an error in the preflight validations to fail fast instead of fail later in the flow.

Start by locating the preflight validations in the create flow and trace how the registry mirror is checked. Confirm the current unauthorized failure path, then verify that a non-public mirror is rejected during preflight rather than later in cluster creation.

Written by the indexing model from the issue text.