feat: upgrade MiniMax default model to M3 (#857)

- Add MiniMax-M3 to the model selection list (set as new default at top)
- Retain MiniMax-M2.7 and MiniMax-M2.7-highspeed
- Remove deprecated MiniMax-M2.5 / M2.5-highspeed
- Update supportsImageInput: M3 supports image input (M2.x stay text-only)
- Update unit tests to reflect new model lineup
- Update example AI_MODEL in CN/EN/JA docs to MiniMax-M3

Co-authored-by: octo-patch <octo-patch@github.com>

docs/ja/ai-providers.md

@@ -222,7 +222,7 @@ MiniMax は 2 つの API 形式をサポートしています：
 
 ```bash
 MINIMAX_API_KEY=your_api_key
-AI_MODEL=MiniMax-M2.7
+AI_MODEL=MiniMax-M3
 ```
 
 オプション設定：