v2.19.0 regression: a plain VP9/AV1 PublishTrack is reported to the server as SVC #1046

Since v2.19.0 (#989), publishing a VP9 or AV1 track with PublishTrack makes the server report it as SVC (codecs[0].video_layer_mode = MULTIPLE_SPATIAL_LAYERS_PER_STREAM). It is a single RTP stream, and the SDK can't publish SVC. With v2.18.1 the same publish is reported as ONE_SPATIAL_LAYER_PER_STREAM.

Cause: #989 made the single-track path always send SimulcastCodecs for video, but it leaves VideoLayerMode unset. The simulcast path sets it to ONE_SPATIAL_LAYER_PER_STREAM. When the mode is unset, the server infers it from the codec and assumes SVC for VP9 and AV1. VP8 and H264 are unaffected.

To reproduce: publish a VP9 or AV1 sample track with PublishTrack, then read the track's video_layer_mode from the server, for example with RoomService.GetParticipant.

Fix: #1045 sets ONE_SPATIAL_LAYER_PER_STREAM in the single-track path, just as the simulcast path already does.