fix(registry): honor credential store when fetching backend images

[doringeman]

The backend image fetcher only read inline auths entries from ~/.docker/config.json, so credentials that docker login stores in the OS keychain were invisible and pulls through an authenticated registry mirror failed with a misleading Docker Hub 403. It now resolves credentials through credential helpers and the credential store, scopes DOCKER_HUB_* environment credentials to Hub's own hosts, and adds an inference.RegistryCredentials hook so Docker Desktop can inject in-process credentials instead of shelling out to a helper.

[sourcery-ai]

Hey - I've left some high level feedback:

• The fake docker-credential helper in TestResolveDigest_AuthenticatedMirror_CredentialStore is implemented as a shell script and skipped on Windows; consider rewriting it in Go so the regression test runs cross-platform and doesn’t depend on /bin/sh being present.
• defaultCredentials currently hard-codes Docker Hub hostnames in isHubHost; if other Hub endpoints or alternate schemes are introduced, it may be safer to centralize or reuse the host normalization logic already present in the authn package to avoid divergence.

Prompt for AI Agents

Please address the comments from this code review:

## Overall Comments
- The fake docker-credential helper in TestResolveDigest_AuthenticatedMirror_CredentialStore is implemented as a shell script and skipped on Windows; consider rewriting it in Go so the regression test runs cross-platform and doesn’t depend on /bin/sh being present.
- defaultCredentials currently hard-codes Docker Hub hostnames in isHubHost; if other Hub endpoints or alternate schemes are introduced, it may be safer to centralize or reuse the host normalization logic already present in the authn package to avoid divergence.

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