[#75705] Use fulfilled_at when mass-assigning price policies

I added scenarios to the `OrderDetail` specs for assigning to price policies other than the current one. Should we expand the controller specs too?
Why did you leave the expiration date nil? I'm pretty sure it always will grab the most recent so this will likely work, but theoretically expire_date should never be nil (and probably should have a validation on it, but I'm not sure now is the time to go down that rabbit hole).