Replaces whisper-server --convert. A zero exit that wrote nothing is an error.
Also fixes the red test's stub, which wrote an empty file and so (correctly)
tripped the no-output check.
Checked against real ffmpeg on iguana with the same argv: AAC .m4a in,
pcm_s16le 16000 Hz mono WAV out, duration preserved.
Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>
