cost_per_second treated a declared-but-zero output_cost_per_second as a real
rate, so the output branch claimed the call and the elif locked out
input_cost_per_second. Every transcription model shipping
output_cost_per_second 0.0 next to a real input rate billed $0, which covers
43 of the 55 per-second entries in the cost map: all 36 deepgram models, both
assemblyai, both elevenlabs scribe, both groq whisper and azure-stt. Custom
deployments pairing the two fields the same way billed $0 as well
Take the output branch only when that rate is actually billable, so a zero
falls through to the input rate. Entries that duplicate one rate into both
fields, whisper-1 among them, keep billing exactly what they bill today
