fix: address code review issues for Task 1

- Add test for ValueError when all simulations completed
- Include warnings in API response for user visibility
- Improve error message validation in tests

api/main.py

@@ -318,6 +318,7 @@ def create_app(
                 status="pending",
                 total_model_days=len(all_dates) * len(models_to_run),
                 message=message,
+                warnings=warnings if warnings else None,
                 **deployment_info
             )