feat(provider): add minimax_cn provider for Chinese endpoint (minimaxi.com)

[soh-tetsu]

Summary

The existing minimax provider points to api.minimax.io (international endpoint). MiniMax also operates a separate Chinese endpoint at api.minimaxi.com which requires its own provider entry.

Problem

Using the generic anthropic_compatible provider with ANTHROPIC_URL=https://api.minimaxi.com/anthropic/v1 fails at activation:

[00:06:52] AnthropicCompatible is now the default provider
[00:06:52] ERROR: Failed to fetch models from Anthropic provider
    Caused by: Failed to fetch models
    Caused by: GET https://api.minimaxi.com/anthropic/v1/models
    Caused by: 404 GET https://api.minimaxi.com/anthropic/v1/models

Forge calls GET {base_url}/models to discover available models, but MiniMax's CN endpoint does not implement that endpoint.

Fix

Add a dedicated minimax_cn provider with a hardcoded model list, bypassing the live /models fetch entirely — the same approach used by minimax (international).

Models included: MiniMax-M2.7, MiniMax-M2.7-highspeed, MiniMax-M2.5, MiniMax-M2.5-highspeed, MiniMax-M2.1, MiniMax-M2.1-highspeed, MiniMax-M2

Usage

forge provider login
# select minimax_cn → enter MINIMAX_API_KEY

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[amitksingh1490]

duplicate #2701