Ceara/aitt 1182b model tests

[cearachew]

Adds model tests for the new aidiff artifact models

To follow the migration PR here: #69511

Links

• Jira: https://codedotorg.atlassian.net/browse/AITT-1182

Testing story

Deployment strategy

Follow-up work

Privacy

Security

Caching

PR Creation Checklist:

• Tests provide adequate coverage
• Privacy impacts have been documented
• Security impacts have been documented
• Code is well-commented
• New features are translatable or updates will not break translations
• Relevant documentation has been added or updated
• User impact is well-understood and desirable
• Follow-up work items (including potential tech debt) are tracked and linked