Stage 3: deploy FastAPI shell as Cloud Run candidate

[anth-volk]

Fixes #3648

Summary

• Add a Cloud Run ASGI runtime for policyengine_api.asgi:app.
• Add Cloud Run deployment helpers, Docker runtime, tagged candidate URL resolution, and traffic promotion scripts.
• Wire staging and production CI so App Engine and Cloud Run deploy on separate tracks, with deployed-service tests before Cloud Run promotion.
• Connect the Cloud Run candidate to the existing production Cloud SQL instance while keeping tests read-only or mocked.
• Add a dedicated Cloud Run runtime service account path and document required IAM.
• Add public Cloud Run health probes and migration request logging for FastAPI-native health routes.
• Keep App Engine/custom API URL traffic unchanged in this PR.

Validation

• FLASK_DEBUG=1 .venv/bin/python -m pytest tests/unit/test_cloud_run_deploy_scripts.py -q
• FLASK_DEBUG=1 .venv/bin/python -m pytest tests/unit/test_asgi_factory.py::test_public_simulation_gateway_health_probe_checks_gateway -q
• FLASK_DEBUG=1 .venv/bin/python -m pytest tests/unit/test_asgi_factory.py::test_public_simulation_gateway_health_probe_reports_failure -q
• ruff check policyengine_api/asgi_factory.py tests/unit/test_asgi_factory.py tests/unit/test_cloud_run_deploy_scripts.py
• bash -n .github/scripts/deploy_cloud_run_candidate.sh
• git diff --check

Notes

The Cloud Run service URL becomes live after candidate validation, but this does not migrate the public App Engine/custom API URL. The existing untracked markdowns/ folder is not part of this PR.

[codecov]

Codecov Report

❌ Patch coverage is 92.50000% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 78.98%. Comparing base (d36050d) to head (cf629b5).
⚠️ Report is 26 commits behind head on master.

Files with missing lines	Patch %	Lines
policyengine_api/asgi_factory.py	92.00%	2 Missing ⚠️
policyengine_api/migration_logging.py	87.50%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3649      +/-   ##
==========================================
+ Coverage   78.89%   78.98%   +0.09%     
==========================================
  Files          72       72              
  Lines        4241     4269      +28     
  Branches      787      790       +3     
==========================================
+ Hits         3346     3372      +26     
- Misses        691      693       +2     
  Partials      204      204              

☔ 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.