Fix auto mode when only OpenRouter is configured

The get_available_models method in ModelProviderRegistry was only checking
for providers with SUPPORTED_MODELS attribute, which OpenRouter doesn't have.
This caused auto mode to fail with "No models available" error when only
OpenRouter API key was configured.

Added special handling for OpenRouter provider to check its _registry
for available models, ensuring auto mode works correctly with OpenRouter.

Added comprehensive tests to verify:
- Auto mode works with only OpenRouter configured
- Model restrictions are respected
- Graceful handling when no providers are available
- No crashes when OpenRouter lacks _registry attribute

providers/registry.py

@@ -183,6 +183,16 @@ class ModelProviderRegistry:
                             continue
 
                         models[model_name] = provider_type
+                elif provider_type == ProviderType.OPENROUTER:
+                    # OpenRouter uses a registry system instead of SUPPORTED_MODELS
+                    if hasattr(provider, "_registry") and provider._registry:
+                        for model_name in provider._registry.list_models():
+                            # Check restrictions if enabled
+                            if restriction_service and not restriction_service.is_allowed(provider_type, model_name):
+                                logging.debug(f"Model {model_name} filtered by restrictions")
+                                continue
+
+                            models[model_name] = provider_type
 
         return models