fix: unblock avatar audio after lost stream trailer

[mikemikimike]

Summary

When an avatar audio stream loses its trailer, DataStreamAudioReceiver can remain blocked on that reader forever even after later audio streams arrive. This change closes the stale reader when a subsequent stream is received, allowing the receiver to emit the pending segment end and process queued audio.

Testing

• uv run --package livekit-agents --with pytest python -m pytest tests/test_datastream_io.py -q
• uv run --package livekit-agents ruff check livekit-agents/livekit/agents/voice/avatar/_datastream_io.py tests/test_datastream_io.py
• git diff --check

[mikemikimike]

Thanks for catching this. Fixed in 0a74af3 by closing the last queued reader when a new stream arrives, while preserving the active-reader fallback. Added a regression test covering an active reader followed by a queued reader and a new stream. Local ruff, format, and diff checks pass; the focused pytest dependency sync was blocked by slow wheel downloads, so CI is being used for the full test verification.