Update Assignment Classes

Set the requesting teacher's classes on this devoir.

The payload is the caller's OWN class list for the devoir, and only that: on a shared devoir (objet commun) every collaborator assigns their own classes to the same document, so the endpoint merges — colleagues' classes are kept untouched, the caller's subset is replaced. Assigning stays possible after the shared freeze (the frozen content is exactly what makes it safe).