tests: Add test for installing from an auth-protected OCI registry

Add a regression test that installs from an auth-protected OCI registry.
The registry requires a bearer token for all /v2/ requests; the client
authenticates via the mock test authenticator and the installation
completes successfully. The test runs for both user and system install
paths.

Assisted-by: Cursor
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Gitea Version: 1.28.0+dev-477-g8b6ad49a5f