fix(voice): complete AgentTask teardown after handoff failures

[IanCollection]

What changed

• guarantee an AgentTask signals inactivity across setup, wait, and handoff teardown failures
• make AgentActivity.aclose() retryable when teardown raises partway through
• have session close retry the current task activity before walking back to its parent

Why

AgentSession waits for an active AgentTask to become inactive before it can continue closing. The event was set only after asynchronous handoff cleanup. If activity close or parent resume raised (including cancellation during setup), the signal was skipped and session shutdown could wait forever. A failed activity close also marked the activity closed before cleanup completed, preventing a later retry.

The inactive signal now covers the full handoff lifecycle, while teardown errors still propagate. A partially failed activity close rolls back its completed flag under the close lock so session shutdown can retry it.

Impact

Session shutdown no longer deadlocks after an AgentTask handoff failure, and partially closed child activities can finish releasing their resources.

Validation

• fail-before regression: AgentSession.aclose() timed out after an injected task-activity close failure
• fail-before regression: cancellation during task activity setup left session close waiting
• pytest tests/test_agent_task_close_race.py tests/test_nested_agent_task.py --unit -q — 5 passed
• broader AgentSession/AgentTask selection — 86 passed
• Ruff check and format check passed
• mypy passed for the changed voice modules

[CLAassistant]

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[devin-ai-integration]

Devin Review found 1 potential issue.

View 2 additional findings in Devin Review.

[devin-ai-integration]

🟡 Session shutdown can stop halfway when a child task's cleanup keeps failing

The retry cleanup of the finished task's activity is awaited without any error handling (await activity.aclose() at livekit-agents/livekit/agents/voice/agent_session.py:1219), so a cleanup error that happens again on the retry aborts the rest of the shutdown.
Impact: If a task's cleanup fails repeatedly, the session never finishes closing — the parent agent is never drained, the close event is never emitted, and the room connection stays open.

Why the retry can abort the whole close path

With the new rollback in AgentActivity.aclose() (livekit-agents/livekit/agents/voice/agent_activity.py:1367-1371), a failed teardown resets _closed = False and re-raises. The new call at agent_session.py:1219 therefore re-runs the same teardown steps; if the underlying failure is deterministic (e.g. rt_session.aclose() always raising), the exception propagates out of the while loop inside _aclose_impl while self._lock is held. _aclose_impl is wrapped in @utils.log_exceptions, so the error is only logged, but everything after the loop is skipped: activity.drain()/aclose() for the parent (agent_session.py:1231-1256), IO detach, self._activity = None, self._started = False, the close event emit, and self._room_io.aclose(). Since _closing_task remains set, _close_soon() will not retry either.

Before this PR the same failing teardown surfaced inside the AgentTask handoff (agent.py:1050) and session close still completed, so this is a new failure mode. Logging the retry failure instead of propagating it preserves the intended "retry idempotent teardown" behaviour without regressing shutdown.

Suggested change

	# The handoff normally closes this activity. Retry idempotently in case
	# its close failed before the parent activity could resume.
	await activity.aclose()
	# The handoff normally closes this activity. Retry idempotently in case
	# its close failed before the parent activity could resume.
	try:
	await activity.aclose()
	except Exception:
	logger.exception(
	"failed to close the AgentTask activity while closing the session"
	)

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