feat: re-acquire mic track when effects require specific audio constraints

[antsukanova]

Summary

Adds constraint handling to LocalAudioStream so audio effects can request specific MediaTrackConstraints on the microphone track. Since Chrome silently ignores applyConstraints() for audio processing properties (chromium#40555809), the handler re-acquires the track via getUserMedia(). Original track settings are saved and restored when the effect releases its constraints on disable/dispose.

Keeping the old track alive while calling getUserMedia() causes Chrome to reuse the existing hardware capture session and silently ignore hardware-level constraints such as echoCancellation. The old track is stopped before getUserMedia() to force a fresh session and ensure all constraints are honored.

What's changed

• addConstraintHandlers — new private method on LocalAudioStream wired up from addEffect(); listens for ConstraintsRequired / ConstraintsReleased events from the effect and calls reacquireInputTrack
• reacquireInputTrack — stops the current track before calling getUserMedia() with the merged baseline + effect constraints; restores mute state and wires the new track into the effect chain
• AppliableAudioConstraints — extended with sampleRate, sampleSize, and channelCount
• filterToSupportedConstraints — new helper that strips keys not recognized by the browser via getSupportedConstraints() before passing baseline settings to getUserMedia()
• effects / loadingEffects / changeOutputTrack — visibility changed from private to protected in LocalStream to allow access from LocalAudioStream
• Failure handling — if getUserMedia() or effect wiring fails, all effects are disposed and Ended is emitted; if the effect is disposed while getUserMedia() is in-flight, the new track is discarded and Ended is emitted to prevent a silent dead stream

Testing

• Enable OFMV → verify autoGainControl: false, noiseSuppression: false, echoCancellation: true
• Disable OFMV → verify original values restored
• BNR enable/disable → no constraint re-acquisition triggered
• Dispose during pending getUserMedia() → new track discarded, Ended emitted

See code change in web-media-effects
You can use the sdk linked branch with these changes to test: agc-fix-branch-test