Move validation logic to the Version class

[bastianallgeier]

Reasoning

We want to be able to validate individual versions before publishing them.

Changelog

Enhancements from previous betas

• New Version::errors() method
• New Version::isValid() method, which is now used in ModelWithContent::isValid() instead of the Form class.

Refactoring

• Switch to PHPUnit Attributes in VersionTest and ModelWithContentTest
• New ModelWithContent::isValid unit tests

Breaking changes

None

Docs

Ready?

• In-code documentation (wherever needed)
• Unit tests for fixed bug/feature
• Tests and CI checks all pass

For review team

• Add changes & docs to release notes draft in Notion

[distantnative]

I would suggest to also add Version::isValid($language).

[bastianallgeier]

@distantnative 👍 while refactoring this, I realized that we have two different ways to validate between ModelWithContent::isValid() and ModelWithContent::errors() 🙈 In ::errors we go through all sections and also collect errors from sections without fields. (e.g. for pages with a max or min) and in ::isValid we only validate the content. Not sure what to do with that and if we should already tackle it here as well.

[distantnative]

@bastianallgeier This is indeed a weird split between ModelWithContent::errors() and ModelWithContent::isValid(). But it has been weird like this before, so no need to fix that right now with this PR.