Currently the unassigned array in output provides an id key matching the input task that we did not manage to fit in. The problem is that we only enforce id values to be unique across same task types (job, pickup or delivery) so in some situations the id information can be ambiguous.

In case of duplicate ids, it's still possible to go through the route steps to see what task types with matching ids are actually included and deduce what is unassigned, but that's definitely not convenient.

We should provide a type key for unassigned tasks, just as for route steps.