feat: add MiniMax-M2.7-highspeed model and MiniMax TTS support

- Add MiniMax-M2.7-highspeed constant to const.py and MODEL_LIST
- Update MinimaxBot default model from MiniMax-M2.1 to MiniMax-M2.7
- Add MinimaxVoice TTS provider (voice/minimax/minimax_voice.py)
  - Supports speech-2.8-hd and speech-2.8-turbo models
  - SSE streaming with hex-decoded audio chunks
  - Reuses MINIMAX_API_KEY
- Register MinimaxVoice in voice factory
- Add unit tests (14 tests, all passing)
- Update README with MiniMax-M2.7-highspeed and TTS configuration

voice/factory.py

@@ -54,4 +54,8 @@ def create_voice(voice_type):
         from voice.tencent.tencent_voice import TencentVoice
 
         return TencentVoice()
+    elif voice_type == "minimax":
+        from voice.minimax.minimax_voice import MinimaxVoice
+
+        return MinimaxVoice()
     raise RuntimeError