Cast time_period to string before posting to simulation gateway

[DTrim99]

Summary

• One-line fix: stringify time_period in the dict produced by SimulationOptions.model_dump() right before simulation_api.run(...) posts it to the gateway.

Why

The simulation gateway (projects/policyengine-api-simulation/src/modal/gateway/models.py in policyengine-api-v2, PR #458 merged 2026-04-17, deployed to prod as v42 on 2026-04-26) tightened its request schema to require:

time_period: Optional[str] = None

The locally-installed policyengine PyPI package's SimulationOptions declares (policyengine.py commit 29f2c1a, 2025-01-28):

TimePeriodType = int
class SimulationOptions(BaseModel):
    time_period: TimePeriodType = Field(...)

So SimulationOptions.model_validate({"time_period": "2027", ...}) coerces "2027" → 2027 (int), and model_dump() re-emits it as int. The gateway then returns:

422 Unprocessable Entity - {"detail":[{"type":"string_type","loc":["body","time_period"],"msg":"Input should be a valid string"}]}

…which surfaces as a 500 to every /<country_id>/economy/<id>/over/<id> request currently. All four state dashboards that depend on the live economy endpoint are blocked.

Approach

Two places to fix:

• B.1 (this PR): cast at the call site in economy_service.py — minimum surface, ships immediately, doesn't require coordinating a policyengine PyPI release.
• B.2 (deferred): add a @field_serializer to SimulationOptions.time_period upstream in PolicyEngine/policyengine.py. Worth doing later as defence-in-depth so any other consumer of model_dump() is also covered, but this PR is sufficient to unblock production.

Test plan

• Verify /us/economy/<id>/over/<id>?region=mo&time_period=2027 no longer returns 500/422 from the gateway after this lands.
• Check NC/UT/SC dashboards (which only read precomputed CSVs) are unaffected — they don't go through this code path.
• No simulation-side regressions: downstream SimulationOptions.time_period is still typed as int internally; we only stringify the JSON body.

🤖 Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 50.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 76.66%. Comparing base (939b716) to head (52ba58c).
⚠️ Report is 9 commits behind head on master.

Files with missing lines	Patch %	Lines
policyengine_api/services/economy_service.py	50.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3522      +/-   ##
==========================================
- Coverage   76.68%   76.66%   -0.02%     
==========================================
  Files          63       63              
  Lines        3444     3446       +2     
  Branches      620      621       +1     
==========================================
+ Hits         2641     2642       +1     
  Misses        629      629              
- Partials      174      175       +1     

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[anth-volk]

LGTM. Note this won't fully fix issue until Docker-related deploy issues are resolved in forthcoming PR(s) from my end.

[anth-volk]

Sorry, actually, just now realizing this doesn't fully solve. Will write in Slack.