ci: use MiniMax-M3 for PR reviews

[SaladDay]

Summary

• switch AI PR review from MiniMax-M2.7-highspeed to MiniMax-M3
• use the MiniMax CN Anthropic-compatible endpoint for the configured Actions API key

Validation

• verified MiniMax-M3 authentication with a minimal request against the CN endpoint
• git diff --check

---

^{Need help on this PR? Tag @codesmith-bot with what you need. Autofix is disabled.}