Fix client not setting msid for camera video tracks (#518)

## Description

Root cause: createSimulcastTransceiverConfig in transceivers.ts was
missing the streams field in its
return object. Audio and non-simulcast video both had it, but simulcast
video did not so the browser
sent the simulcast track with no stream ID in the SDP (a=msid:- ...),
causing the server to see streams:
  [] for the video track while audio got the correct stream ID. 

## Motivation and Context

Why is this change required? What problem does it solve? If it fixes an
open
issue, please link to the issue here.

## Documentation impact

- [ ] Documentation update required
- [ ] Documentation updated [in another PR](_)
- [x] No documentation update required

## Types of changes

- [x] Bug fix (non-breaking change which fixes an issue)
- [ ] New feature (non-breaking change which adds functionality)
- [ ] Breaking change (fix or feature that would cause existing
functionality to
      not work as expected)

Author: Gawor270

packages/webrtc-client/src/tracks/transceivers.ts

@@ -99,6 +99,7 @@ const createSimulcastTransceiverConfig = (
     // in other case lower resolution encoding can get
     // higher max_bitrate
     sendEncodings: calculateSimulcastEncodings(encodings, maxBandwidth),
+    streams: trackContext.stream ? [trackContext.stream] : [],
   };
 };
 

packages/webrtc-client/tests/methods/addTrackMethod.test.ts

@@ -1,7 +1,10 @@
+import { FakeMediaStreamTrack } from 'fake-mediastreamtrack';
 import { expect, it } from 'vitest';
 
-import { WebRTCEndpoint } from '../../src';
+import { Variant, WebRTCEndpoint } from '../../src';
+import { TrackContextImpl } from '../../src/internal';
 import { deserializePeerMediaEvent, serializeServerMediaEvent } from '../../src/mediaEvent';
+import { createTransceiverConfig } from '../../src/tracks/transceivers';
 import { createConnectedEventWithOneEndpoint, mockTrack } from '../fixtures';
 import { mockMediaStream, mockRTCPeerConnection } from '../mocks';
 
@@ -51,6 +54,29 @@ it('Adding track updates internal state', () => {
   expect(localEndpoint.tracks.size).toBe(1);
 });
 
+it('Simulcast transceiver config includes the stream', () => {
+  const stream = { id: 'test-stream-id' } as MediaStream;
+  const videoTrack = new FakeMediaStreamTrack({ kind: 'video' });
+
+  const trackContext = new TrackContextImpl(
+    { id: 'endpoint-1', type: 'webrtc', metadata: undefined, tracks: new Map() },
+    'track-1',
+    undefined,
+    {
+      enabled: true,
+      enabledVariants: [Variant.VARIANT_LOW, Variant.VARIANT_MEDIUM, Variant.VARIANT_HIGH],
+      disabledVariants: [],
+    },
+  );
+  trackContext.track = videoTrack;
+  trackContext.stream = stream;
+  trackContext.maxBandwidth = 0;
+
+  const config = createTransceiverConfig(trackContext);
+
+  expect(config.streams).toEqual([stream]);
+});
+
 it('Adding track before being accepted by the server throws error', async () => {
   // Given
   mockRTCPeerConnection();