fix(config): scope library/ prefix to Docker Hub

[louis-kauer]

What issue type does this pull request address?
/kind bugfix

What does this pull request do? Which issues does it resolve?
resolves #4021

Gates the library/ namespace injection in Image.String() on the registry actually being Docker Hub (docker.io or index.docker.io). Previously, any non-empty registry plus a slashless repository got library/ prepended, producing invalid references like ghcr.io/library/alpine:3.20. Round-trip via ParseImageRef is preserved.

Please provide a short message that should be published in the vcluster release notes

Fixed `Image.String()` injecting a `library/` namespace prefix on non-Docker-Hub registries, producing invalid references like `ghcr.io/library/alpine:3.20`. The prefix is now only applied when the registry is Docker Hub.

What else do we need to know?
Tests pass locally (go test ./config/...). Updates one existing test case (omit repo but not registry is library) that encoded the bug as expected output, plus six new cases covering Docker Hub canonical/alias and non-Docker-Hub registries. No API or config changes.

E2E Tests

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Additional test suites

none