Add MiniMax hosted video generation tool

[octo-patch]

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

Summary

• add a hosted text-to-video and image-to-video CLI with global_en and cn_zh API regions
• add asynchronous task polling, local image data URL support, request validation, and MP4 download
• register and document the tool and skill, including environment setup and Codex migration coverage
• add mocked unit coverage for payloads, regional endpoints, polling, API errors, and downloads

Checks

• .venv/bin/python -m pip check
• .venv/bin/python -m compileall -q tools/minimax_video.py tests/test_minimax_video.py
• .venv/bin/python -m unittest discover -s tests -p 'test_minimax_video.py'
• .venv/bin/python tools/minimax_video.py --help
• .venv/bin/python -m json.tool _internal/toolkit-registry.json
• .venv/bin/python scripts/migrate_to_codex.py --repo-root . --dry-run
• git diff --check