[codex] Prepare simulation service for policyengine 0.13.0

[MaxGhenis]

Summary

• pin the simulation project and Modal image to the expected next policyengine 0.x release
• add a regression test to keep the pyproject.toml and Modal image pins in sync

Validation

• /Users/maxghenis/PolicyEngine/policyengine-api-v2/projects/policyengine-api-simulation/.venv/bin/pytest tests/test_policyengine_dependency_source.py
• python3 -m py_compile projects/policyengine-api-simulation/src/modal/app.py projects/policyengine-api-simulation/tests/test_policyengine_dependency_source.py
• black --check projects/policyengine-api-simulation/src/modal/app.py projects/policyengine-api-simulation/tests/test_policyengine_dependency_source.py

Notes

• This PR is intentionally in advance of the policyengine 0.x release. I am inferring 0.13.0 from the current 0.12.1 pin plus the merged minor bump in policyengine.py 0.x.
• CI is expected to fail until policyengine==0.13.0 is actually available on PyPI.
• The policyengine.py 0.x versioning workflow is currently failing on checkout auth, so publishing that release is separately blocked until that workflow issue is fixed.