ci: add package verification check (#897)

## Summary

Adds an end-to-end check that the built sdist and wheel install cleanly,
ship the expected sources and data files, and that `import apify` works
— via the shared
[apify/workflows/python-package-check](https://github.com/apify/workflows/pull/296)
action.

Wired into every PR via `_check_package.yaml`, and into both stable and
beta release workflows so the exact artifact about to hit PyPI is
verified first.

## Verified locally

Built `dist/` and ran the verification — all checks pass
(`apify==3.4.0`, sdist + wheel both install cleanly into a fresh Python
3.14 venv).