Fix RI retirement income subtraction per-person cap

[MaxGhenis]

Closes #7022

Summary

• Rhode Island retirement income subtraction now correctly applies the cap per person instead of per tax unit
• Joint filers can now receive up to $40K (2024) or $100K (2025) in subtractions when both spouses have qualifying pension income

Changes

• Changed ri_retirement_income_subtraction.py to apply min_(taxable_pension, p.cap) per person before summing
• Added test cases for joint filers with per-person cap verification

Test

• Added integration tests verifying:
  • 2024: Joint filers each get $20K cap ($40K total)
  • 2025: Joint filers each get $50K cap ($100K total)

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 (2dcf1ae).
⚠️ Report is 447 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff              @@
##             main     #7295       +/-   ##
============================================
+ Coverage   72.72%   100.00%   +27.27%     
============================================
  Files        3757         1     -3756     
  Lines       54115        16    -54099     
  Branches      267         0      -267     
============================================
- Hits        39353        16    -39337     
+ Misses      14742         0    -14742     
+ Partials       20         0       -20     

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

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