Add MiniMax provider coverage

[octo-patch]

Reason: add target provider/model to existing provider registry

Summary

• Register MiniMax-M3 and MiniMax-M2.7 pricing, including MiniMax-M3 standard and priority input-length tiers.
• Keep cost estimates accurate per request and support LiteLLM-prefixed model names.
• Document global and China OpenAI- and Anthropic-compatible endpoints in both provider guides.

Checks

• python -m pytest tests/ -v (92 passed)
• python -m ruff check corecoder tests
• python -m compileall -q corecoder tests
• python -m build
• python -m twine check dist/*
• Local request-path capture for /v1/chat/completions and /anthropic/v1/messages