Route API v1 through the certified PolicyEngine bundle

[anth-volk]

Fixes #3706

Summary

• replace separate US/UK/core model pins with policyengine[models]==4.18.3
• derive API v1 model versions from the installed .py bundle manifest
• send policyengine_version to the simulation gateway while preserving legacy model_version compatibility
• omit default dataset values so the simulation gateway resolves the certified bundle dataset
• update deployment guards and automated bundle-update workflow for .py bundle routing

Testing

• UV_CACHE_DIR=/private/tmp/uv-cache uv run ruff format --check .
• UV_CACHE_DIR=/private/tmp/uv-cache uv run python scripts/run_quality_guards.py
• UV_CACHE_DIR=/private/tmp/uv-cache POLICYENGINE_DB_PASSWORD=test FLASK_DEBUG=1 uv run pytest tests/unit/test_constants.py tests/unit/libs/test_simulation_api_modal.py tests/contract/test_simulation_gateway_contract.py tests/unit/services/test_economy_service.py tests/unit/test_cloud_run_deploy_scripts.py -q
• .github/request-simulation-model-versions.sh -py 4.18.3 -us 1.729.0 -uk 2.89.2
• LATEST_OVERRIDE=4.18.4 .github/scripts/update-policyengine-package.sh --dry-run

[codecov]

Codecov Report

❌ Patch coverage is 80.00000% with 20 lines in your changes missing coverage. Please review.
✅ Project coverage is 78.87%. Comparing base (84d1a0f) to head (0fc959c).
⚠️ Report is 8 commits behind head on master.

Files with missing lines	Patch %	Lines
policyengine_api/constants.py	73.33%	5 Missing and 7 partials ⚠️
policyengine_api/services/economy_service.py	87.17%	2 Missing and 3 partials ⚠️
policyengine_api/libs/simulation_api_modal.py	81.25%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3708      +/-   ##
==========================================
- Coverage   78.98%   78.87%   -0.11%     
==========================================
  Files          72       71       -1     
  Lines        4269     4318      +49     
  Branches      790      804      +14     
==========================================
+ Hits         3372     3406      +34     
- Misses        693      698       +5     
- Partials      204      214      +10     

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.