Add MiniMax image generation tool

[octo-patch]

Reason: add target image capability using existing tool/provider pattern

Summary

• Add a MiniMax-backed generate_image tool to the tool-use lesson.
• Register the tool in the existing TOOLS schema and TOOL_HANDLERS dispatch map.

Checks

• python3 -m py_compile s02_tool_use/code.py
• secret scan on git diff

[octo-patch]

Follow-up audit completed. I made the image tool configurable for both regional API hosts, synchronized the three lesson docs, diagram, and generated web data, and added a focused request-contract test. I ran python -m pytest tests -q (25 passed, 32 subtests), tsc --noEmit, and npm run build.