Default US metadata to Populace

[MaxGhenis]

Summary\n- advertise the certified Populace bundle as the default US economy dataset in metadata\n- keep legacy CPS and enhanced CPS as explicit non-default options\n- add a regression test so app-v2 continues to omit dataset= for default Populace runs\n\nThis complements PolicyEngine/populace#228, which makes the sparse L0+refit build the default release artifact. API economy runs already omit the concrete data field for dataset=default, allowing the simulation gateway/policyengine.py bundle to resolve the certified Populace dataset and apply state/CD row filters.\n\n## Tests\n- uv run ruff format policyengine_api/country.py tests/unit/services/test_metadata_service.py\n- uv run ruff check policyengine_api/country.py tests/unit/services/test_metadata_service.py\n- uv run pytest tests/unit/services/test_metadata_service.py tests/unit/services/test_economy_service.py::TestEconomicImpactSetupOptions::TestSetupSimOptions tests/unit/libs/test_simulation_api_modal.py::TestSimulationAPIModal::TestRun::test__given_api_v1_default_bundle_payload__then_posts_gateway_contract_body

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 78.81%. Comparing base (63bb169) to head (158817f).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #3715   +/-   ##
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  Coverage   78.81%   78.81%           
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  Files          71       71           
  Lines        4320     4320           
  Branches      804      804           
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  Hits         3405     3405           
  Misses        702      702           
  Partials      213      213           

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