feat: Add Modal simulation API alongside GCP Workflows

[anth-volk]

Fixes #3064
Requires PolicyEngine/policyengine-api-v2#383

Summary

Adds Modal simulation API support alongside the existing GCP Workflows implementation, allowing the economy report endpoint to use either backend via environment variable configuration.

Changes

New Files

• policyengine_api/libs/simulation_api_modal.py - HTTP client for Modal API
• policyengine_api/libs/simulation_api_factory.py - Factory to select GCP or Modal backend
• tests/unit/libs/test_simulation_api_modal.py - Unit tests for Modal client (20 tests)
• tests/unit/libs/test_simulation_api_factory.py - Unit tests for factory (7 tests)
• tests/fixtures/libs/simulation_api_modal.py - Test fixtures for Modal client
• tests/fixtures/libs/simulation_api_factory.py - Test fixtures for factory

Modified Files

• policyengine_api/constants.py - Added status constants for GCP and Modal
• policyengine_api/services/economy_service.py - Uses factory, handles both status formats
• setup.py - Added httpx>=0.27.0 dependency
• tests/fixtures/services/economy_service.py - Added Modal test fixtures
• tests/unit/services/test_economy_service.py - Added Modal status tests (5 tests)

How to Enable Modal

# Enable Modal backend
USE_MODAL_SIMULATION_API=true

# Optional: Override Modal API URL
SIMULATION_API_URL=https://policyengine--policyengine-simulation-web-app.modal.run

By default (USE_MODAL_SIMULATION_API unset or false), the existing GCP Workflows backend is used.

Test plan

• Unit tests pass for SimulationAPIModal (20 tests)
• Unit tests pass for factory function (7 tests)
• Unit tests pass for Modal status handling in EconomyService (5 tests)
• Manual integration test with Modal backend passes (Utah state reform)
• CI integration tests pass (requires Modal to have matching package version deployed)

Notes

• The model_version field is mapped to version for the Modal API
• data_version is removed as Modal doesn't use it
• No database schema changes - execution_id column works with both ID formats
• Integration tests require the local policyengine_us version to match a version deployed on Modal

🤖 Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 91.30435% with 8 lines in your changes missing coverage. Please review.
✅ Project coverage is 71.05%. Comparing base (9c84311) to head (b1dfe06).
⚠️ Report is 10 commits behind head on master.

Files with missing lines	Patch %	Lines
policyengine_api/libs/simulation_api_modal.py	86.44%	7 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3065      +/-   ##
==========================================
+ Coverage   70.25%   71.05%   +0.80%     
==========================================
  Files          55       57       +2     
  Lines        2353     2436      +83     
  Branches      333      339       +6     
==========================================
+ Hits         1653     1731      +78     
- Misses        639      644       +5     
  Partials       61       61              

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