feat(minimax): add minimax-cn-oauth provider for China-region OAuth

[goingforstudying-ctrl]

Summary

Add a separate minimax-cn-oauth provider mirroring the existing minimax-oauth, but pointing at China-region endpoints.

Fixes #25542

Changes

plugins/model-providers/minimax/init.py

• New ProviderProfile: minimax_cn_oauth
• base_url: https://api.minimaxi.com/anthropic
• auth_type: oauth_external
• display_name: "MiniMax China (OAuth)"

hermes_cli/auth.py

• New PROVIDER_REGISTRY entry: minimax-cn-oauth
• portal_base_url: MINIMAX_OAUTH_CN_BASE
• inference_base_url: MINIMAX_OAUTH_CN_INFERENCE
• auth_type: oauth_minimax

hermes_cli/models.py

• Add minimax-cn-oauth to PROVIDER_GROUPS[minimax]
• Add curated model list for minimax-cn-oauth: MiniMax-M3, MiniMax-M2.7, MiniMax-M2.7-highspeed

Motivation

The _minimax_oauth_login helper already accepts region=cn and swaps China URLs. But no separate minimax-cn-oauth provider was registered, so China users had no OAuth path.

Testing

• hermes model picker should show 4 MiniMax entries including MiniMax China OAuth
• hermes auth add minimax-cn-oauth should trigger OAuth flow

Checklist

• Code follows project style
• Changes are minimal and focused
• Fixes the reported issue Add China-region (minimax-cn) OAuth provider — code already supports it, but not exposed #25542

[goingforstudying-ctrl]

Hi, just checking in on this PR. It has been a few days since the last update. Please let me know if there is anything else needed from my side to move this forward. Thanks for your time.