Partner API rejects an unknown assignee when updating a contact

What’s changed

Updating a contact with PUT /api/partner/people/{id} now checks the assignee the same way creating one does:

• An assignedUserId, assignedPondId or assignedGroupId that isn’t an active user, pond or group on the team returns 422 with an errorMessage that names the field. The contact is left unchanged.
• Before, the request returned 200 and kept the old assignment, so an integration had no way to tell the assignment hadn’t happened.
• A user whose account has been deleted is no longer accepted as an assignee on create or update.

Sending an assignee field as null or "" still clears the assignment. Sending one field blank alongside another that has a value doesn’t skip the check: an unknown assignedPondId next to a blank assignedUserId still returns 422.