Route API v1 simulation traffic through the Stage 5 entrypoint

[anth-volk]

Goal

Make API v1 capable of calling the separate authenticated Cloud Run Simulation Entrypoint while preserving the existing direct Modal gateway path for controlled rollout and rollback.

Scope

• introduce a generic Simulation API client with explicit old-gateway and Cloud Run entrypoint configuration;
• select the active backend through validated startup configuration;
• preserve compatibility imports and existing simulation behavior;
• deploy candidate API v1 revisions without automatically changing traffic;
• provide an operator-run, revision-validating command for manual 0/5/25/50/100 rollout and rollback.

Acceptance criteria

• API v1 fails startup for invalid or incomplete selected-entrypoint configuration;
• both direct-Modal and Cloud Run entrypoint modes pass focused tests;
• jobs submitted before rollback remain pollable through the compatible service boundary;
• no GitHub Actions workflow sets API v1 traffic percentages.