feat: Add include_district_breakdowns parameter for US national simulations

[anth-volk]

Fixes #3073

Summary

• Adds include_district_breakdowns query parameter to the economy endpoint for US national simulations
• When include_district_breakdowns=true is passed for US national-level simulations (country_id=us, region=us), sets the dataset to national-with-breakdowns
• The Modal gateway routes this to run_national_with_breakdowns, which spawns parallel simulations and returns results with congressional_district_impact data

Changes

• economy_routes.py: Added handling for include_district_breakdowns query parameter
• economy_service.py: Added PASSTHROUGH_DATASETS set containing accepted keywords (national-with-breakdowns, national-with-breakdowns-test) and updated _setup_data() to pass these directly to the simulation API
• test_economy_service.py: Added tests for passthrough dataset behavior

Test plan

• Verify existing economy service tests pass
• Test US national simulation with include_district_breakdowns=true
• Verify response includes congressional_district_impact data

🤖 Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 75.00000% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 70.73%. Comparing base (3f04db8) to head (e1c61f2).
⚠️ Report is 5 commits behind head on master.

Files with missing lines	Patch %	Lines
policyengine_api/routes/economy_routes.py	50.00%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #3074   +/-   ##
=======================================
  Coverage   70.73%   70.73%           
=======================================
  Files          57       57           
  Lines        2436     2443    +7     
  Branches      339      341    +2     
=======================================
+ Hits         1723     1728    +5     
- Misses        653      654    +1     
- Partials       60       61    +1     

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