Keep SSI SGA screen separate from disability criteria

[MaxGhenis]

Summary

• keeps meets_ssi_disability_criteria as a static/data-backed disability criteria indicator before SGA
• applies ssi_engaged_in_sga in is_ssi_disabled so the SGA screen remains dynamic in microsimulations and reforms
• adds coverage that data-provided disability criteria still fails SSI disabled status when the person is engaged in SGA

Tests

• uv run policyengine-core test policyengine_us/tests/policy/baseline/gov/ssa/ssi/meets_ssi_disability_criteria.yaml policyengine_us/tests/policy/baseline/gov/ssa/ssi/is_ssi_disabled.yaml -c policyengine_us
• uv run python policyengine_us/tests/test_batched.py policyengine_us/tests/policy/baseline/gov/ssa/ssi --mode per-subdir
• uv run ruff format/check on touched Python files

Notes

This follow-up fixes the contract needed by policyengine-us-data: the dataset should impute the disability-status signal, while PolicyEngine-US applies the current SGA rule separately.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (289712f) to head (903c656).
⚠️ Report is 9 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff              @@
##             main     #8365       +/-   ##
============================================
+ Coverage   70.38%   100.00%   +29.61%     
============================================
  Files        4791         2     -4789     
  Lines       69837        22    -69815     
  Branches      345         0      -345     
============================================
- Hits        49153        22    -49131     
+ Misses      20684         0    -20684     

Flag	Coverage Δ
unittests	100.00% <100.00%> (+29.61%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ 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.