Fix staged deploy fallback for simulation gateway URL

[anth-volk]

Summary

• rebase the staged auth client branch onto current origin/master
• default staged App Engine deploys to the stable simulation gateway URL when SIMULATION_API_URL is unset or blank
• stop treating SIMULATION_API_URL as required deploy-time configuration while preserving environment overrides
• add coverage for the blank-env fallback in the simulation gateway client

Validation

• uv run ruff check policyengine_api/libs/simulation_api_modal.py gcp/export.py tests/unit/libs/test_simulation_api_modal.py
• uv run pytest --noconftest tests/unit/libs/test_simulation_api_modal.py
• bash -n .github/scripts/validate_app_engine_deploy_env.sh

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 77.00%. Comparing base (01d5bc1) to head (08ba60c).
⚠️ Report is 5 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #3504   +/-   ##
=======================================
  Coverage   77.00%   77.00%           
=======================================
  Files          63       63           
  Lines        3418     3418           
  Branches      617      617           
=======================================
  Hits         2632     2632           
  Misses        612      612           
  Partials      174      174           

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