[Task]: Add test ensuring all promoted classes have unique ids

[diox]

Description

For activities (and maybe others) we have an assertion ensuring we can't re-use the same action. We should have that for PromotedClass too, ensuring we can't re-use an id or api_name that is already used.

Acceptance Criteria

Milestones/checkpoints

Beta Give feedback

1. A test ensuring all promoted classes have unique ids exist

Checks

• If I have identified that the work is specific to a repository, I have removed "repository:addons-server" or "repository:addons-frontend"

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