Keep only the invariant the fix owns (an envelope dict logs its serialized
size on the concurrent path); the plain-string control was already covered
by the pre-existing behaviour and doubled the file. Import the AIAgent stub
and fake tool-call shapes from test_start_order_gate.py instead of copying
them, so the concurrent-executor stub has one home.
Part of #112095. Salvage of #112104 (@kokhlo).
teknium1
efdf766cad