gpt-4o-mini-tts and gpt-4o-mini-transcribe slugs now point to 2025-12-15 snapshots

Updated the gpt-4o-mini-tts and gpt-4o-mini-transcribe slugs to point to the 2025-12-15 snapshots. If you need the previous model snapshots, use gpt-4o-mini-tts-2025-03-20 and gpt-4o-mini-transcribe-2025-03-20. We currently recomend using gpt-4o-mini-transcribe over gpt-4o-transcribe for the best results.