_update_config_for_provider() was called immediately after provider
selection for zai, kimi-coding, minimax, minimax-cn, and anthropic —
before model selection happened. Since the gateway re-reads config.yaml
per-message, this created a race where the gateway would pick up the
new provider but still use the old (incompatible) model name.
Capture selected_base_url in each provider block, then call
_update_config_for_provider() once, after model selection completes,
right before save_config(). The in-memory _set_model_provider() calls
stay in place so the config object remains consistent during setup.
Closes#1182
36dd7a3e8d