Add MiniMaxProvider with built-in support for MiniMax's OpenAI-compatible
API (MiniMax-M2.7, MiniMax-M2.5, MiniMax-M2.5-highspeed models).
Changes:
- Add MiniMaxProvider class implementing ModelProvider interface
- Add MiniMax model routing in OpenAIChatCompletionsModel for litellm
- Export MiniMaxProvider from agents __init__.py
- Add provider documentation (docs/providers/minimax.md)
- Add usage example (examples/model_providers/minimax_example.py)
- Add MiniMax to README provider list
- Add 18 unit tests and 3 integration tests