feat(audio): add AudioManager microphone mute mode API

[hiroshihorie]

Description

Adds AudioManager.setMicrophoneMuteMode / getMicrophoneMuteMode with a LiveKit-owned MicrophoneMuteMode enum (voiceProcessing, restart, inputMixer), mirroring the Swift SDK's AudioManager.microphoneMuteMode.

Background

On iOS/macOS the AVAudioEngine-based ADM defaults to voiceProcessing muting, which plays the platform's mute/unmute sound effect. This is the LiveKit-level counterpart of flutter-webrtc/flutter-webrtc#2105 (see also flutter-webrtc/flutter-webrtc#2098): apps can select inputMixer or restart to mute silently.

await AudioManager.instance.setMicrophoneMuteMode(MicrophoneMuteMode.inputMixer);

The mode applies to LiveKit's own mute path: disabling a published local audio track drives the engine-level microphone mute in webrtc (WebRtcVoiceSendChannel::MuteStream → adm->SetMicrophoneMute). For the fastest silent mute toggling, combine inputMixer with AudioCaptureOptions(stopAudioCaptureOnMute: false) — documented on the API.

API notes

• No flutter_webrtc types on the public surface: the enum is LiveKit-owned and named to match the Swift SDK (restart, not restartEngine); the mapping to flutter_webrtc happens internally.
• No-op on non-Apple platforms, including for unknown, so set(await get()) round-trips safely in cross-platform code.
• Engine-wide state; recommended to set once before connecting.

Dependencies

⚠️ Requires a flutter_webrtc release containing flutter-webrtc/flutter-webrtc#2105. Draft until that ships and the flutter_webrtc pin is bumped here (local development uses a gitignored pubspec_overrides.yaml).

Testing

• dart analyze lib test clean, all 314 tests pass.