feat: Add MiniMax Anthropic-compatible API support (minimax-api)

Add --auth-choice minimax-api for direct MiniMax API usage at
https://api.minimax.io/anthropic using the anthropic-messages API.

Changes:
- Add applyMinimaxApiConfig() function with provider/model config
- Add minimax-api to AuthChoice type and CLI options
- Add handler and non-interactive support
- Fix duplicate minimax entry in envMap
- Update live test to use anthropic-messages API
- Add 11 unit tests covering all edge cases
- Document configuration in gateway docs

Test results:
- 11/11 unit tests pass
- 1/1 live API test passes (verified with real API key)

Co-Authored-By: Claude <noreply@anthropic.com>

src/commands/auth-choice-options.ts

@@ -101,6 +101,10 @@ export function buildAuthChoiceOptions(params: {
   // Token flow is currently Anthropic-only; use CLI for advanced providers.
   options.push({ value: "minimax-cloud", label: "MiniMax M2.1 (minimax.io)" });
   options.push({ value: "minimax", label: "Minimax M2.1 (LM Studio)" });
+  options.push({
+    value: "minimax-api",
+    label: "MiniMax API (platform.minimax.io)",
+  });
   if (params.includeSkip) {
     options.push({ value: "skip", label: "Skip for now" });
   }