Function architect_audio_transcription_multipart 

pub(super) fn architect_audio_transcription_multipart(
    boundary: &str,
    model_id: &str,
    filename: &str,
    media_type: &str,
    audio: &[u8],
) -> Option<Vec<u8>>

Builds a bounded multipart/form-data payload for external LLM vendor audio transcription.