RelayAdapter.on_interrupt(session_key, chat_id) bridges a connector-delivered
mid-turn /stop into the existing interrupt_session_activity path, setting the
per-session _active_sessions Event and clearing typing — cancelling exactly the
targeted session's turn without touching siblings (mirrors test_stop_thread_
sibling isolation). Transport.send_interrupt carries the gateway-side egress to
the connector for socket-owner routing.
Phase 1, Task 1.4 of the gateway-relay plan.
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