#14106: fix(model): hardcoded fallback for claude-opus-4-6 when no template exists

## Summary - Add hardcoded fallback for `anthropic/claude-opus-4-6` in `resolveAnthropicOpus46ForwardCompatModel()` when no template model exists in the registry, preventing "Unknown model" errors - Fix MiniMax OpenAI-compatible base URL from `api.minimax.chat` to `api.minimaxi.chat` (the correct domain with an 'i'), which caused all MiniMax completions to fail with HTTP 401 ## Test plan - [x] Existing model resolution tests pass (11/11) - [x] MiniMax models-config test updated and passes - [x] Image tool test updated and passes - [x] Verified MiniMax fallback works end-to-end (Anthropic rate limit → MiniMax responds) 🤖 Generated with [Claude Code](https://claude.com/claude-code)