test(editor): Unit tests for assistant store and new design system components (no-changelog)

[MiloradFilipovic]

Summary

• Unit tests for AI Assistant store (note that some functionality is not tested here since it's covered in end-to-end tests. Did this to avoid having to mock a lot of things in unit tests)
• Added snapshot tests for new design-system components

Related Linear tickets, Github issues, and Community forum posts

Closes ADO2-74 Closes ADO2-65

Review / Merge checklist

• [ ] PR title and summary are descriptive. (conventions)
• [ ] Docs updated or follow-up ticket created.
• [ ] Tests included.
• [ ] PR Labeled with release/backport (if the PR is an urgent fix that needs to be backported)