fix: prevent wait_for_idle cancellation from poisoning user turns

[Excelius-Wang]

Fixes #6911.

Summary

• Shield the end-of-turn and user-turn-completion tasks observed by wait_for_idle().
• Wait for a predecessor user-turn task without propagating cancellation in either direction.
• Preserve non-cancellation exceptions raised by the predecessor.
• Add regression tests for cancellation isolation and successor recovery.

Why

wait_for_idle() is an observer of SDK-owned tasks. Cancelling the observer currently cancels the task it is awaiting. A later user turn then awaits that cancelled predecessor and is cancelled before processing, so one observer cancellation can poison subsequent turns.

Testing

• pytest tests/test_wait_for_idle_cancellation.py -q — 3 passed
• pytest tests/test_tools.py --unit -q — 114 passed
• ruff format --check livekit-agents/livekit/agents/voice/agent_activity.py tests/test_wait_for_idle_cancellation.py
• ruff check livekit-agents/livekit/agents/voice/agent_activity.py tests/test_wait_for_idle_cancellation.py
• mypy -p livekit.agents.voice.agent_activity

[devin-ai-integration]

✅ Devin Review: No Issues Found

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