fix(voice): don't hold session.run() open on update_agent handoffs with long-lived on_enter

[k-zaher]

What

session.run() deadlocks when a function tool calls session.update_agent(new_agent) and the new agent's on_enter awaits an AgentTask (or TaskGroup) that needs a future user turn to complete.

The chain: update_agent() → _update_activity_task (watched by the active RunResult so the handoff result lands before run completion) → _update_activity(agent) with the default wait_on_enter=True → awaits the new agent's on_enter → which awaits an AgentTask that needs the next user turn. But the driver can't send the next turn until run() returns — a circular wait. The run hangs until an external timeout cancels everything (surfacing as ToolError: AgentTask ... is cancelled).

Both other activity-transition paths already guard against exactly this:

• session.start() passes wait_on_enter=False (agent_session.py:916), which is why the examples/survey pattern (agent on_enter awaiting a whole TaskGroup) works when the agent is the starting agent.
• AgentTask.__await_impl passes wait_on_enter=False with a comment describing this exact deadlock: "on_enter may spawn nested AgentTasks that require user input, but session.run() can't return until all watched handles complete — creating a circular wait."

update_agent() was the only path still waiting. Entering the same agent that works via session.start() through an update_agent() handoff deadlocks — reproduced on 1.6.4, 1.6.5, and 1.6.6.

Fix

Pass wait_on_enter=False in _update_activity_task, matching the other two paths.

Test

tests/test_update_agent_long_on_enter.py — FakeLLM-scripted: a greeter tool calls update_agent() into an agent whose on_enter awaits an AgentTask needing another user turn; asserts run() returns on the handoff turn and the task completes on the following turn. Deadlocks (times out) without the fix; passes with it.

Validation:

• tests/test_agent_session.py, test_nested_agent_task.py, test_audio_recognition_handoff.py, test_agent_task_close_race.py, test_drain_timeout.py, test_tool_results_preserved_on_interruption.py, test_user_turn_exceeded.py, test_speech_handle_exception.py, test_agent_update_options.py + the new test: 116 passed.
• examples/survey/test_survey_agent.py: 5 passed (live LLM).
• A production voice agent using this shape (greeter agent routing via update_agent() to an agent whose on_enter drives a multi-task loop): its full offline suite of ~1000 tests passes against this patch; on current releases every workflow test deadlocks.

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[devin-ai-integration]

✅ Devin Review: No Issues Found

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[longcw]

thanks for the pr! could sign the CLA before we can merge

[k-zaher]

I have signed the CLA but the bot is not picking up the updated status for some reason