Pipeline.run() used asyncio.gather, so if one loop raised, the failing
coroutine propagated while the sibling loops kept running detached; aclose()
in the finally then closed a source/stt out from under a still-live loop.
Switch to asyncio.TaskGroup so a failing loop cancels+awaits the siblings
before teardown. Add a regression test asserting an error in the conversation
loop cancels the perception loop and still closes every source.
3cc262ed18