fix(rtc): keep pending read when AudioMixer stream times out

[RaphaelFakhri]

Description

AudioMixer races each iterator.next() against streamTimeoutMs. On a timeout it moves on, and the next pass calls next() again. The first read is still pending, so the frame it later resolves with has no consumer and is dropped. Every timeout leaves another abandoned read, so audio that resumes after a silent period loses its first frames.

This change stores the pending read per stream and awaits it again on the next pass instead of starting a new one. The entry is cleared when the read settles, when the read rejects, and in removeStream.

Testing

Added does not drop a frame that arrives after a read timeout to audio_mixer.test.ts. It feeds a stream whose first frame arrives after the read timeout and expects both frames to come out of the mixer.

• Without the change: 8 passed, 1 failed (received [], expected [111, 222]).
• With the change: 9 passed.

Command: pnpm prebuild && npx vitest run src/audio_mixer.test.ts in packages/livekit-rtc.

A patch changeset for @livekit/rtc-node is included.

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🦋 Changeset detected

Latest commit: dc78d58

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@livekit/rtc-node	Patch

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