feat: add MiniMax as new LLM provider

Add support for MiniMax M2.5 series models via OpenAI-compatible API:
- Add minimax_fn() to Provider class with configurable base URL
- Support both international and China mainland endpoints
- Update documentation in README.md and README_CHS.md
- Add MINIMAX_API_KEY and MINIMAX_BASE_URL to .env.example
- Add unit tests for MiniMax provider

Made-with: Cursor

.env.example

@@ -10,4 +10,8 @@ DEEPSEEK_API_KEY='xxxxx'
 OPENROUTER_API_KEY='xxxxx'
 TOGETHER_API_KEY='xxxxx'
 GOOGLE_API_KEY='xxxxx'
-ANTHROPIC_API_KEY='xxxxx'
+ANTHROPIC_API_KEY='xxxxx'
+MINIMAX_API_KEY='xxxxx'
+# Optional: MiniMax API base URL (default: https://api.minimax.io/v1)
+# For mainland China users: https://api.minimaxi.com/v1
+# MINIMAX_BASE_URL='https://api.minimax.io/v1'