Allow raw CPS source artifact through publication contract

[MaxGhenis]

Summary

• keep enforcing the leaf-input export contract for final simulation datasets
• exempt the raw CPS source artifact from the no-computed-policyengine-us-variable check during publication validation
• add regression coverage showing raw CPS can publish source variables while enhanced CPS still rejects computed exports

Testing

• uv run pytest tests/unit/test_dataset_validation.py tests/unit/test_upload_completed_datasets.py
• uv run ruff format policyengine_us_data/utils/dataset_validation.py policyengine_us_data/storage/upload_completed_datasets.py tests/unit/test_dataset_validation.py tests/unit/test_upload_completed_datasets.py
• uv run ruff check policyengine_us_data/utils/dataset_validation.py policyengine_us_data/storage/upload_completed_datasets.py tests/unit/test_dataset_validation.py tests/unit/test_upload_completed_datasets.py