A child agent spawned from a graph node now carries its parent's session
abort signal, so a user interrupt is observable from every sync wait in
the agent tree. Background agent__spawn children are unchanged.
run_child_agent_with_graph_inputs also guards the child's own supervisor
with a disarmable CancelOnDrop: if the future is dropped mid-flight or
the child fails before its normal cleanup, its spawned subagents are
cancelled instead of running on invisibly.
Dark-Alex-17
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