Pro 6272 fix relationships

[ValJed]

https://linear.app/apostrophecms/issue/PRO-6272/relationships-fields-are-broken

Summary

What are the specific steps to test this change?

• Adds a validation step for relationship fields, it wasn't done.
• Fixes the relation ship fields not having the data when coming from the relationship modal.
• Fixes watch on checkedDocs in the slat list not being reactive and not seeing updated relationship fields.
• Minor lint

Cypress tests HERE 🟠

What kind of change does this PR introduce?

• [X] Bug fix
• [ ] New feature
• [ ] Refactor
• [ ] Documentation
• [ ] Build-related changes
• [ ] Other

Make sure the PR fulfills these requirements:

• [X] It includes a) the existing issue ID being resolved, b) a convincing reason for adding this feature, or c) a clear description of the bug it resolves
• [X] The changelog is updated
• [ ] Related documentation has been updated
• [ ] Related tests have been updated

[linear[bot]]

PRO-6272 Relationships fields are broken