Async-delegation completions (delegate_task(background=true)) and
background-process completions (terminal notify_on_complete) re-enter the
originating session as internal MessageEvents. When the session was busy,
_handle_active_session_busy_message treated them like a user TEXT message and
the default busy_input_mode='interrupt' aborted the active turn (and sent a
'Interrupting current task' ack) — the opposite of the design invariant that a
completion surfaces as a new turn only when idle.
Short-circuit internal events to return False so the base adapter queues them
silently (it already excludes internal events from debounce), cascading them as
the next turn after the current one finishes.
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