* 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