test: replace live r8.im registry with local test registry in integration tests (#2816)
Use registry-seed with crane.Copy to seed base images into a local test
registry instead of hitting the live r8.im registry, which is flaky in CI.
- Add registry-seed harness command supporting both local and external sources
- Convert build_base_image_sha, build_cog_version_match, build_python313_base_image
to seed python:3.12/3.13-slim from Docker Hub via crane
- Convert torch_cuda_baseimage to use cog debug with a dummy seeded image
- Fix name.Insecure missing from ParseReference in build.go (consistent with
pkg/registry/)