Enable US districts

[anth-volk]

Fixes #2961

Summary

This PR enables US Congressional district-level simulations in the PolicyEngine API. It adds support for running economy/macro simulations at the congressional district level, alongside the existing state and national levels.

Key Changes

• Congressional Districts Data Module (policyengine_api/data/congressional_districts.py): New module containing metadata for all 435 US Congressional districts, including district codes, state associations, and validation utilities
• Region Handling (policyengine_api/services/economy_service.py):
  • Added _setup_region() to properly prefix regions (e.g., ca → state/ca, TX-14 → congressional_district/TX-14)
  • Added _validate_us_region() to validate US states and congressional districts
  • Restored _setup_data() to handle NYC special pooled CPS dataset requirement
• API Metadata (policyengine_api/country.py, policyengine_api/constants.py): Congressional district metadata is now included in the /metadata endpoint response
• Package Update (setup.py): Upgraded policyengine dependency to >=0.7.0 to support district-level simulations

Region Format

The API now supports the following region formats for US simulations:

• National: us
• State: ca, tx, ny (automatically prefixed to state/ca, etc.)
• Congressional district: TX-14, CA-37 (automatically prefixed to congressional_district/TX-14, etc.)
• NYC: nyc (uses pooled 3-year CPS dataset)

Test Plan

• Unit tests for congressional districts validation and lookup functions
• Unit tests for _setup_region() method
• Unit tests for _validate_us_region() method
• Unit tests for _setup_data() method (NYC handling)
• Updated existing _setup_sim_options() tests

[codecov]

Codecov Report

❌ Patch coverage is 95.65217% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 74.87%. Comparing base (10067e3) to head (1c10af2).
⚠️ Report is 25 commits behind head on master.

Files with missing lines	Patch %	Lines
policyengine_api/services/economy_service.py	88.46%	1 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2962      +/-   ##
==========================================
+ Coverage   74.09%   74.87%   +0.77%     
==========================================
  Files          53       54       +1     
  Lines        1965     2026      +61     
  Branches      254      268      +14     
==========================================
+ Hits         1456     1517      +61     
+ Misses        460      459       -1     
- Partials       49       50       +1     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[anth-volk]

Waiting to merge this until we have an accompanying front-end PR ready.