feat: add MiniMax as third LLM provider in Chapter 10

[octo-patch]

Summary

Adds MiniMax as a third LLM provider alongside Google Gemini and GitHub Models in the AI Agent for DevOps tutorial (Chapter 10).

MiniMax exposes an OpenAI-compatible API, so it reuses ChatOpenAI from langchain-openai with a custom base_url — the same pattern already established for GitHub Models. This gives readers another provider option they can switch to with a single LLM_PROVIDER=minimax environment variable.

Changes (10 files, 543 additions)

• New src/models/minimax.py — MiniMaxModel wrapper with temperature clamping (MiniMax requires > 0)
• Updated src/models/factory.py — added minimax to provider dispatch
• Updated src/models/__init__.py — exports MiniMaxModel
• Updated src/config.py — added MINIMAX_API_KEY, MINIMAX_MODEL, MINIMAX_ENDPOINT config vars with validation
• Updated .env.example — documented MiniMax env vars
• Updated app.py — sidebar now shows MiniMax provider label
• Updated README.md — added MiniMax to provider table and project structure
• Updated 10-building-complex-agent-with-actions.md — added MiniMax code example, config, and validation in tutorial text
• New tests/test_minimax.py — 21 unit tests (model init, temperature clamping, factory, config validation, exports)
• New tests/test_minimax_integration.py — 3 integration tests (chat completion, tool binding, factory)

Models supported

Model	Context	Notes
MiniMax-M2.7 (default)	1M tokens	Latest flagship model
MiniMax-M2.7-highspeed	1M tokens	Faster variant

Test plan

• All 21 unit tests pass (python -m pytest tests/test_minimax.py)
• All 3 integration tests pass with live API key
• Verify LLM_PROVIDER=minimax works with streamlit run app.py
• Verify existing Gemini/GitHub providers still work unchanged