Add end-to-end fixtures covering major package and registry scenarios

[CasperKristiansson]

Summary

Create a suite of sample repositories or fixtures that exercise Docker Hub, GHCR, ECR, tox, poetry, pipenv, asdf, and pyenv flows to validate full action runs.

Why

Holistic fixtures help catch integration regressions before release and serve as documentation for contributors. They also support the new roadmap work around registries and multi-environment support.

What needs to happen

• Assemble representative fixtures (or sample repos) covering the listed ecosystems.
• Automate running the action against each fixture in CI, capturing outputs for regression checks.
• Where possible, reuse existing fixture infrastructure but expand coverage.
• Document how to run the fixtures locally and interpret results.

References

• Roadmap quality item: "End-to-end fixtures".

Acceptance criteria

• CI can execute the action against each fixture scenario and fail on regressions.
• Contributors have documented steps for maintaining fixtures.
• README or docs reference the new fixtures.