gpt-realtime-mini and gpt-audio-mini slugs now point to 2025-12-15 snapshots

Updated the gpt-realtime-mini and gpt-audio-mini slugs to point to the 2025-12-15 snapshots. If you need the previous model snapshots, use gpt-realtime-mini-2025-10-06 and gpt-audio-mini-2025-10-06.