test: refactor CreateCatalogHandler tests to use a real registry.

[fabriziosestito]

Right now, the CreateCatalogHandler tests use a mocked registry client. That makes things painful because we have to mock every single request for different cases.

• Let’s switch the tests to use a real registry instead (either our ghcr.io registry or a testcontainer one works).
• Also check the private registry test, as it looks like it’s still mocked, so we’re not really testing auth at the moment.