Function build_webm_transcode_args 

pub fn build_webm_transcode_args(
    input: &Path,
    output: &Path,
    has_audio: bool,
) -> Vec<String>

Build the gst-launch argv for the MP4 → WebM transcode. Split out so tests assert the pipeline shape without spawning gst. The Opus audio leg is present only when has_audio (see transcode_to_webm for why).

Shape: filesrc ! decodebin name=d webmmux name=mux ! filesink d. ! queue ! videoconvert ! vp9enc ! mux. [d. ! queue ! audioconvert ! audioresample ! opusenc ! mux.]