Replace Exercise In Assignment

Swap one exercise of the devoir for one or more of the teacher's own.

What the chat's « coupe cet exercice en 4 » lands on: the new exercises take the slot of the original (same position, same order), the original leaves the devoir with its per-exercise policies, and the students' sessions get the same splice. The new exercises inherit the settings of the one they replace (retry count, chat share) so a split does not reset what the teacher had configured. The ids are teacher exercises the caller already owns (the chat just created them for this), so unlike add_exercises_to_assignment nothing is copied.