feat: add MiniMax LLM provider support (M2.7 default)

[octo-patch]

Summary

Add MiniMax as a first-class LLM provider for MetaGPT via its OpenAI-compatible API.

Changes

• Add LLMType.MINIMAX enum value in llm_config.py
• Register MINIMAX in OpenAILLM provider routing in openai_api.py
• Add temperature clamping for MiniMax (requires > 0)
• Add token costs and context window limits for MiniMax-M1, M2.5, M2.5-highspeed, and M2.7 (default recommended)
• Add MiniMax config example in config2.example.yaml
• Add MiniMax usage example in README

Models Supported

Model	Context Window	Notes
MiniMax-M2.7	204K	Default recommended - latest model
MiniMax-M2.5	204K	Previous generation
MiniMax-M2.5-highspeed	204K	Fast variant
MiniMax-M1	1M	Legacy

Configuration

llm:
  api_type: "minimax"
  model: "MiniMax-M2.7"
  base_url: "https://api.minimax.io/v1"
  api_key: "YOUR_MINIMAX_API_KEY"

Testing

• Verified MiniMax-M2.7 model responds correctly via API
• Token counter and context window values validated programmatically
• LLMType enum verified