Fix CT Social Security benefit adjustment

[MaxGhenis]

Closes #7019

Summary

• CT Social Security benefit adjustment now uses the actual federal taxable Social Security amount (tax_unit_taxable_social_security) instead of 85% of gross Social Security
• The previous implementation incorrectly used gross_ss * 0.85 for Line E of the worksheet
• Per CT-1040 Instructions, Line E should be the "Taxable amount of Social Security benefits reported on your 2024 federal Social Security Benefits Worksheet, Line 18"

Changes

• Updated ct_social_security_benefit_adjustment.py to use tax_unit_taxable_social_security instead of tax_unit_social_security * 0.85
• Updated test expectations to reflect the correct calculation
• Added integration test case from issue CT Social Security benefit adjustment uses 85% of gross SS instead of actual federal taxable SS #7019 (MFJ, both age 75)

Test

• All existing tests pass
• New test verifies the fix matches TaxAct calculations:
  • CT SS Adjustment: $38,802 (not $61,505)
  • CT AGI: $43,544 (not $20,841)
  • CT Tax: $332 (not $0)

Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (66ccff2) to head (edac79e).
⚠️ Report is 439 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff              @@
##             main     #7301       +/-   ##
============================================
+ Coverage   72.72%   100.00%   +27.27%     
============================================
  Files        3757         1     -3756     
  Lines       54115        21    -54094     
  Branches      267         0      -267     
============================================
- Hits        39353        21    -39332     
+ Misses      14742         0    -14742     
+ Partials       20         0       -20     

Flag	Coverage Δ
unittests	100.00% <100.00%> (+27.27%)	⬆️

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.