Too bad you hadn't merely opened the bolt all the way. That way, it would not have fired - I think.

Similar scenario happened to me some time ago with a Mauser that had a binding side safety - cinched down & pulling the trigger resulted in 1/2 a click. Loosening off the guard screws 1/8th turn would allow the pin to drop and fire the rifle. Lifting the bolt first to straight up position, prevented the rifle from being able to be fired. I"m not sure that would prevent the lee from firing (cock on closing action), but do think it might.

One could test that with an empty chamber- just to make sure and be prepared it this every happens again.