fix: prevent model/provider mismatch when switching providers during active gateway (#1183)

When _update_config_for_provider() writes the new provider and base_url
to config.yaml, the gateway (which re-reads config per-message) can pick
up the change before model selection completes. This causes the old model
name (e.g. 'anthropic/claude-opus-4.6') to be sent to the new provider's
API (e.g. MiniMax), which fails.

Changes:
- _update_config_for_provider() now accepts an optional default_model
  parameter. When provided and the current model.default is empty or
  uses OpenRouter format (contains '/'), it sets a safe default model
  for the new provider.
- All setup.py callers for direct-API providers (zai, kimi, minimax,
  minimax-cn, anthropic) now pass a provider-appropriate default model.
- _setup_provider_model_selection() now validates the 'Keep current'
  choice: if the current model uses OpenRouter format and wouldn't work
  with the new provider, it warns and switches to the provider's first
  default model instead of silently keeping the incompatible name.

Reported by a user on Home Assistant whose gateway started sending
'anthropic/claude-opus-4.6' to MiniMax's API after running hermes setup.

hermes_cli/auth.py

@@ -1541,8 +1541,20 @@ def detect_external_credentials() -> List[Dict[str, Any]]:
 # CLI Commands — login / logout
 # =============================================================================
 
-def _update_config_for_provider(provider_id: str, inference_base_url: str) -> Path:
-    """Update config.yaml and auth.json to reflect the active provider."""
+def _update_config_for_provider(
+    provider_id: str,
+    inference_base_url: str,
+    default_model: Optional[str] = None,
+) -> Path:
+    """Update config.yaml and auth.json to reflect the active provider.
+
+    When *default_model* is provided the function also writes it as the
+    ``model.default`` value.  This prevents a race condition where the
+    gateway (which re-reads config per-message) picks up the new provider
+    before the caller has finished model selection, resulting in a
+    mismatched model/provider (e.g. ``anthropic/claude-opus-4.6`` sent to
+    MiniMax's API).
+    """
     # Set active_provider in auth.json so auto-resolution picks this provider
     with _auth_store_lock():
         auth_store = _load_auth_store()
@@ -1576,6 +1588,15 @@ def _update_config_for_provider(provider_id: str, inference_base_url: str) -> Pa
     else:
         # Clear stale base_url to prevent contamination when switching providers
         model_cfg.pop("base_url", None)
+
+    # When switching to a non-OpenRouter provider, ensure model.default is
+    # valid for the new provider.  An OpenRouter-formatted name like
+    # "anthropic/claude-opus-4.6" will fail on direct-API providers.
+    if default_model:
+        cur_default = model_cfg.get("default", "")
+        if not cur_default or "/" in cur_default:
+            model_cfg["default"] = default_model
+
     config["model"] = model_cfg
 
     config_path.write_text(yaml.safe_dump(config, sort_keys=False))