feat: Add allow_listing_models

• Add allow_listing_models configuration flag to VLLM provider to control model listing behavior
• Implement allow_listing_models() method across all providers with default implementations in base classes
• Prevent HTTP requests to /v1/models endpoint when allow_listing_models=false for improved security and performance
• Fix unit tests to include allow_listing_models method in test classes and mock objects

llama_stack/providers/utils/inference/model_registry.py

@@ -100,6 +100,9 @@ class ModelRegistryHelper(ModelsProtocolPrivate):
     async def should_refresh_models(self) -> bool:
         return False
 
+    async def allow_listing_models(self) -> bool:
+        return True
+
     def get_provider_model_id(self, identifier: str) -> str | None:
         return self.alias_to_provider_id_map.get(identifier, None)