feat: add minimax-api-key-cn option for China API endpoint

- Add 'minimax-api-key-cn' auth choice for Chinese users
- Reuse existing --minimax-api-key CLI option
- Use MINIMAX_CN_API_BASE_URL (https://api.minimaxi.com/anthropic)
- Similar to how moonshot supports moonshot-api-key-cn

Tested: build ✅, check ✅, test ✅

src/commands/onboard-auth.ts

@@ -38,7 +38,9 @@ export {
 } from "./onboard-auth.config-core.js";
 export {
   applyMinimaxApiConfig,
+  applyMinimaxApiConfigCn,
   applyMinimaxApiProviderConfig,
+  applyMinimaxApiProviderConfigCn,
   applyMinimaxConfig,
   applyMinimaxHostedConfig,
   applyMinimaxHostedProviderConfig,
@@ -92,6 +94,7 @@ export {
   KIMI_CODING_MODEL_ID,
   KIMI_CODING_MODEL_REF,
   MINIMAX_API_BASE_URL,
+  MINIMAX_CN_API_BASE_URL,
   MINIMAX_HOSTED_MODEL_ID,
   MINIMAX_HOSTED_MODEL_REF,
   MOONSHOT_BASE_URL,