Broadcast AbstractRNG like a scalar

[nickrobinson251]

• closes AbstractRNG should be a broadcast scalar #31071
• i think this is in-keeping with the de-facto heuristic on how things broadcast, which i think of as "something that's obviously not a collection broadcasts like a scalar, as long as there's a reasonable use-case for that behaviour"
• ideally we'd test this on a new/test-only AbstractRNG, but i wasn't sure exactly what would need to be implemented and if it'd be worth it... so this is just tested on the two subtypes definited in the module :)