feat(agent): support MiniMax image references

[octo-patch]

Reason: Raven's image tool does not send reference images to the MiniMax image-generation API.

Summary

• Route the configured MiniMax global and China API bases to image generation.
• Map local paths, data URLs and public reference URLs to character subjects while retaining batching, output naming and workspace restrictions.
• Resolve MiniMax credentials explicitly and guard generated image downloads.
• Document configuration for image-01 and image-01-live.

Type

• Fix
• Feature
• Docs
• CI / tooling
• Refactor
• Other

Verification

• Relevant tests pass locally: uv run pytest tests/test_media_gen_tool.py tests/test_config_provider_passthrough.py -x -q (121 passed).
• Relevant lint / type checks pass locally: uv run ruff check raven/agent/tools/media_gen.py raven/config/schema.py tests/test_media_gen_tool.py.
• User-facing docs or screenshots are updated when needed: docs/minimax-images.md.

Risk

• Security impact considered
• Backward compatibility considered
• Rollback path is clear for risky changes

The new route activates only for the two configured MiniMax regional API bases. Reference images are character subjects, not mask edits. API credentials are excluded from image downloads, which retain the public-target checks. Verification uses mocked HTTP responses; no paid generation request was made. Revert this commit to remove the route.

Related Issues

#307