fix(voice): block new user turns immediately on update_agent() to prevent transition delay

[Panmax]

Summary

Bug: When update_agent() is called, the old agent can still accept and process new user turns (including ambient noise) during the transition window, delaying the new agent's on_enter() by seconds.

Root cause: The async transition task (drain() → _pause_scheduling_task()) doesn't run immediately, leaving a window where the old activity's scheduling task accepts new turns and triggers full LLM → TTS pipeline.

Fix: Introduce a lightweight _new_turns_blocked flag that is set synchronously in update_agent() to immediately block new user turns, while keeping _scheduling_paused untouched for _pause_scheduling_task() to function correctly.

This avoids the issue where directly setting _scheduling_paused = True would cause _pause_scheduling_task() to early-return and skip await asyncio.shield(self._scheduling_atask), which is needed to wait for in-flight speech tasks to complete.

Changes

• agent_activity.py: Add _new_turns_blocked flag, checked at all user-turn entry points (_on_generation_created, on_preemptive_generation, on_end_of_turn, _on_user_turn_completed, _on_user_input_complete)
• agent_session.py: update_agent() sets _new_turns_blocked = True instead of _scheduling_paused = True

What is NOT changed (intentionally)

• _pause_scheduling_task() early-return guard — must only check _scheduling_paused
• _schedule_speech(force=True) — tool responses during drain still work
• _scheduling_task exit condition — only exits on formal pause signal
• say()/generate_reply() routing — programmatic speech still routes to current activity during transition

Test plan

• Call session.update_agent(new_agent) while there is ambient noise — new agent's on_enter() should fire promptly without the old agent processing a stray turn
• Verify drain() properly awaits in-flight speech tasks before completing the transition
• Verify normal agent transitions still work correctly
• Verify that in-progress turns at the time of update_agent() complete without interruption

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