Add MiniMax (https://platform.minimaxi.com/) as a built-in chat model
provider with M2.7, M2.7-highspeed, M2.5, and M2.5-highspeed models.
MiniMax provides OpenAI-compatible API, so it works seamlessly with
the existing OpenAIBase and ChatOpenAI integration paths. Users only
need to set MINIMAX_API_KEY to start using MiniMax models.
Changes:
- Add MiniMax to DEFAULT_CHAT_MODEL_PROVIDERS (models.py)
- Add MINIMAX_API_KEY to .env.template
- Add MiniMax provider icon and frontend registration
- Update model-config docs with MiniMax entry
- Add 22 unit tests + 3 integration tests
