* API: Add `createdAt` and `updatedAt` to api/users/lookup
In the past, we have added both `updatedAt` (#19004) and `createdAt` (#19475) to /api/users/:id
Turns out, api/users/lookup uses the same DTO for both. This fixes the serialization of both `createdAt` and `updatedAt`for this endpoint.
Also, adds a test to ensure no further regressions.
* Updated API documentation