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

README.md

@@ -240,7 +240,7 @@ provider_server_address = # Typically ignored or can be left blank when is_local
 | Hugging Face | `huggingface`   | No     | Use models from Hugging Face Inference API.       | [huggingface.co/settings/tokens](https://huggingface.co/settings/tokens) |
 | TogetherAI   | `togetherAI`    | No     | Use various open-source models via TogetherAI API.| [api.together.ai/settings/api-keys](https://api.together.ai/settings/api-keys) |
 | OpenRouter   | `openrouter`    | No     | Use OpenRouter Models| [https://openrouter.ai/](https://openrouter.ai/) |
-| MiniMax      | `minimax`       | No     | Use MiniMax M2.5 series models (e.g., MiniMax-M2.5).| [platform.minimax.io](https://platform.minimax.io/user-center/basic-information) |
+| MiniMax      | `minimax`       | No     | Use MiniMax models (e.g., MiniMax-M2.7, MiniMax-M2.5).| [platform.minimax.io](https://platform.minimax.io/user-center/basic-information) |
 
 *Note:*
 *   We advise against using `gpt-4o` or other OpenAI models for complex web browsing and task planning as current prompt optimizations are geared towards models like Deepseek.