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

models/minimax/minimax_bot.py

@@ -20,7 +20,7 @@ class MinimaxBot(Bot):
     def __init__(self):
         super().__init__()
         self.args = {
-            "model": conf().get("model") or "MiniMax-M2.1",
+            "model": conf().get("model") or "MiniMax-M2.7",
             "temperature": conf().get("temperature", 0.3),
             "top_p": conf().get("top_p", 0.95),
         }