Serialize date metadata values

[MaxGhenis]

Summary

• Convert date and datetime values in country metadata to ISO strings before serving metadata JSON
• Add a unit test for date serialization in PolicyEngineCountry._json_safe

Tests

• env -u UV_FROZEN uv run ruff format --check policyengine_api/country.py tests/unit/test_country.py
• env -u UV_FROZEN uv run python -m py_compile policyengine_api/country.py tests/unit/test_country.py
• env -u UV_FROZEN uv run pytest -q tests/unit/test_country.py::TestCountryJsonSafety (blocked locally: POLICYENGINE_DB_PASSWORD is required when policyengine_api.data initializes)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 77.83%. Comparing base (1fd9618) to head (e957564).
⚠️ Report is 4 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3571      +/-   ##
==========================================
+ Coverage   77.81%   77.83%   +0.01%     
==========================================
  Files          65       65              
  Lines        3877     3880       +3     
  Branches      697      698       +1     
==========================================
+ Hits         3017     3020       +3     
  Misses        683      683              
  Partials      177      177              

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