Add MiniMax as alternative LLM provider

[octo-patch]

Summary

• Add MiniMax as an alternative LLM provider in the RAG chain server get_llm() factory
• MiniMax provides an OpenAI-compatible API with models like MiniMax-M2.7 (1M context) and MiniMax-M2.5-highspeed (204K context, speed-optimized)
• Uses ChatOpenAI from langchain-openai with MiniMax base URL — no new proprietary SDK required
• Temperature clamping to [0, 1] range for MiniMax API compatibility

Changes

File	Change
RAG/src/chain_server/utils.py	Add minimax branch in get_llm() factory using ChatOpenAI
RAG/src/chain_server/configuration.py	Update model_engine help text to list minimax
RAG/src/chain_server/requirements.txt	Add langchain-openai>=0.0.6 dependency
docs/change-model.md	Add MiniMax usage documentation with env vars
RAG/src/chain_server/tests/	Add 15 unit tests + 3 integration tests

Usage

Test plan

• 15 unit tests covering MiniMax provider creation, temperature clamping, parameter forwarding, config defaults, error handling
• 2 unit tests verifying NVIDIA AI endpoints path is unchanged
• 3 integration tests verifying real API calls (chat completion, streaming, M2.5-highspeed model)
• Docker compose integration test with APP_LLM_MODELENGINE=minimax