Route API v1 simulations through the Stage 5 entrypoint

[anth-volk]

Summary

• introduce a generic SimulationAPIClient and retain compatibility aliases for the former Modal-specific client;
• keep the old Modal gateway URL and the new Cloud Run Simulation Entrypoint URL independently configured;
• select the active simulation entrypoint through validated startup configuration;
• add no-traffic Cloud Run candidate support and an operator-run, revision-validating rollout/rollback command;
• ensure GitHub Actions never sets API v1 traffic percentages.

Why

API v1 needs to adopt the separate Stage 5 simulation service boundary without coupling the migration to a one-way cutover. Keeping both targets selectable allows controlled qualification, cross-polling, and immediate rollback while the new Cloud Run entrypoint continues to use the existing Modal gateway underneath.

Impact

The default remains the direct old-gateway path until an operator deliberately deploys and promotes a revision configured for the Cloud Run entrypoint. Existing imports remain available through explicit compatibility aliases.

Root cause

The previous client and deployment configuration assumed that API v1 called the Modal-hosted gateway directly. That made the simulation service boundary inseparable from the Modal hosting implementation and did not provide a guarded, reversible Stage 5 cutover mechanism.

Validation

• 233 focused client, migration, deployment, contract, and economy tests passed during implementation.
• changed ASGI gateway-health probe tests passed.
• the migration contract guard passed.
• startup validation covers missing, partial, and invalid selected-entrypoint configuration.
• deployment-script tests confirm traffic percentages remain operator controlled.

Fixes #3775

[codecov]

Codecov Report

❌ Patch coverage is 97.20670% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 80.71%. Comparing base (e004986) to head (bff2ffc).
⚠️ Report is 10 commits behind head on master.

Files with missing lines	Patch %	Lines
policyengine_api/libs/simulation_entrypoint.py	96.81%	3 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3776      +/-   ##
==========================================
+ Coverage   80.03%   80.71%   +0.67%     
==========================================
  Files          70       74       +4     
  Lines        4342     4422      +80     
  Branches      809      822      +13     
==========================================
+ Hits         3475     3569      +94     
+ Misses        648      623      -25     
- Partials      219      230      +11     

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