When using combined image:tag@digest references, parse_repository_tag
returns (repo, "tag@digest"). The Docker SDK and API don't accept
tag@digest in the tag parameter, causing "invalid tag format" errors.
This fix:
1. Adds build_pull_arguments() helper function to _util.py
2. Uses the helper in both pull_image implementations
3. When tag contains @ (but isn't a pure digest), passes the full
reference as the repository/fromImage parameter instead of splitting
Tested formats:
- portainer/portainer-ee:2.35.0-alpine@sha256:abc...
- ghcr.io/gethomepage/homepage:v1.7@sha256:abc...
- localhost:5000/myapp:v2.0@sha256:abc...
The existing filter_images_by_tag already handles tag@digest for
lookups, so find_image continues to work correctly.
Includes comprehensive unit tests for build_pull_arguments().
