feat: upgrade MiniMax default model to M2.7

- Add MiniMax-M2.7 and MiniMax-M2.7-highspeed to supported models list
- Update docstring to list M2.7 models before M2.5
- Update README/README_CHS model references
- Add unit tests for M2.7 and M2.7-highspeed models
- Keep all previous models as alternatives

sources/llm_provider.py

@@ -416,11 +416,13 @@ class Provider:
     def minimax_fn(self, history, verbose=False):
         """
         Use MiniMax API to generate text via OpenAI-compatible interface.
-        
+
         Supported models:
+        - MiniMax-M2.7: Latest flagship model with enhanced reasoning and coding
+        - MiniMax-M2.7-highspeed: High-speed version of M2.7 for low-latency scenarios
         - MiniMax-M2.5: Peak performance model (~60 tps), 204,800 context window
         - MiniMax-M2.5-highspeed: Same performance, faster (~100 tps)
-        
+
         Note: temperature must be in range (0.0, 1.0], default is 1.0
         """
         load_dotenv()