Update tests away from split policyengine-us-data state/district URIs

[MaxGhenis]

PolicyEngine/populace#237 removed release generation for one-file-per-state and one-file-per-district artifacts. The replacement path is the single certified Populace national H5 plus the standard region/scoping machinery in policyengine.py, not direct hf://policyengine/policyengine-us-data/states/... or districts/... files.

GitHub code search still finds split-H5 references in this repo:

• projects/policyengine-api-simulation/tests/test_dataset_uri.py
• projects/policyengine-api-simulation/tests/test_simulation_output_builder.py
• projects/policyengine-api-simulation/fixtures/gateway/test_endpoints.py

Migration checklist:

• Stop constructing or documenting policyengine-us-data/states/*.h5 and policyengine-us-data/districts/*.h5 dataset URIs.
• Use the certified Populace default dataset through policyengine.py, with region filters/scoping for state and congressional-district analyses.
• Add or update a regression test so future state/CD runs do not depend on split regional H5 files.
• Keep historical/generated output unchanged unless it is actively used by the app or docs.

[anth-volk]

Working on this as part of a different deployment issue