Good question, while we probably can't just ring up a tool manufacturer and ask, we can derive it with some accuracy:
Maximum socket size I can find for 1/2" is 36mm, that translates to an M24 fixing: http://stainlessautomotivefastenings.co.uk/pdfs/HEXAGONHEADIDENTIFIER.pdf
Standard torque figures for an M24 using a 12.9 grade fixing (strongest grade) is 926 lb ft
So I think it would reasonable to say that, even with a tiny factor of safety, a 1/2 drive extension should be able to handle 1000 lb ft
Applying the same logic to 3/8", that would be a 22mm (M14) fixing, which equates to 177 lb ft. So 130 lb ft plus seizure could well exceed the limit for 3/8" hardware.