Set TfStereo when publishing stereo audio sources

[truemagic-coder]

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• I have read and followed the principles regarding breaking changes, testing, and code quality.

PR description

Fixes #1018.

When a LocalAudioTrack is created from a two-channel AudioSource, the publication path does not currently include AudioTrackFeature::TfStereo in AddTrackRequest.audio_features.

As a result, the server is not told that the source is stereo.

This PR:

• advertises TfStereo for two-channel native audio sources;
• leaves mono sources unchanged;
• does not infer stereo from arbitrary multichannel sources;
• preserves TfPreconnectBuffer behavior and composes it with TfStereo;
• leaves non-audio publication behavior unchanged;
• adds focused regression coverage using real NativeAudioSource values;
• adds the required patch changeset for livekit and livekit-ffi.

NativeAudioSource can represent more than two channels, but TfStereo is a stereo flag rather than a multichannel layout. Automatic stereo advertisement is therefore limited to exactly two channels.

Breaking changes

None.

MSRV

No change.

Testing

Tests cover:

• one-channel sources do not advertise TfStereo;
• two-channel sources advertise TfStereo;
• three-channel sources do not automatically advertise TfStereo;
• TfStereo and TfPreconnectBuffer compose correctly;
• non-audio publication behavior remains unchanged;
• the complete livekit package test suite;
• workspace formatting.

Verification

The exact submitted commit was independently verified with FalseGreen against a frozen verification contract.

• exactly two channels advertise TfStereo;
• mono does not advertise TfStereo;
• more than two channels do not imply stereo;
• stereo composes with preconnect buffering;
• non-audio publication behavior remains unchanged;
• the complete livekit package test suite and workspace formatting pass;
• the required changeset retains patch bumps for livekit and livekit-ffi;
• 7/7 verification criteria passed;
• verified source: 1f76ed000a2d65749c4008a24e7715c1eeca29a7.

The verification is scoped to these criteria and does not claim broader correctness of the LiveKit Rust SDK.

Async

No runtime or async behavior changes.

Assisted-by: Codex

[CLAassistant]

All committers have signed the CLA.

[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no potential bugs to report.

View in Devin Review to see 2 additional findings.