Add tests for NewCommand skeleton package resolution

[roble]

This pull request updates how the NewCommand resolves the skeleton package version for new Saucebase applications, ensuring it always installs the latest stable release instead of accidentally installing a development version. It also introduces comprehensive tests for this logic. The main changes are grouped below:

Improvements to Skeleton Version Resolution:

• The command now fetches the latest published release of the skeleton from Packagist and pins the install to that version, preventing accidental installation of dev-main when offline or when the latest version can't be determined. If offline, it falls back to a numeric constraint defined by the new SKELETON_FALLBACK constant (^2.0). [1] [2]
• The latestVersion method now accepts a package name parameter and queries Packagist dynamically, rather than being hardcoded for the installer package.

Testing Enhancements:

• Added NewCommandTest with a testable subclass to verify that the skeleton is pinned to the latest stable release, correctly falls back when offline, and respects command options. This ensures robust coverage of the new version resolution logic.