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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