Add MiniMax as a fourth TTS provider alongside OpenAI, ElevenLabs, and
Microsoft. Registers a SpeechProviderPlugin in the existing minimax
extension with config resolution, directive parsing, and Talk Mode
support. Hex-encoded audio response from the T2A v2 API is decoded to
MP3.
Closes#52720
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>