The CLI routes user input typed while the agent is running into
``_interrupt_queue`` (separate from ``_pending_input``) so the explicit
interrupt path can opt to deliver them as a single combined message.
That path only drains the queue when ``busy_input_mode == "interrupt"``
AND a ``pending_message`` was acknowledged.
If the agent's turn finishes naturally (no interrupt fires), any
messages typed during the turn stay stuck in ``_interrupt_queue``
forever. Subsequent ``Enter`` presses route input to the same blocked
queue and the CLI appears to hang. Original report: lunarnexus in
The fix restores the post-turn drain that was originally part of
drain off as "worth its own review" and never re-landed it; the user-
visible regression is that any non-interrupt-mode user typing during
a turn is silently dropped.
Implementation: extract the drain to a small helper
``_drain_interrupt_queue_to_pending_input`` matching the existing
``_maybe_continue_goal_after_turn`` style. ``process_loop``'s
``finally`` block calls it once per turn after the status-line refresh
and before goal continuation (so re-queued user input preempts an
auto-continuation prompt). The helper swallows ``Exception`` so it
can never break the main loop.
Addresses #20271.
