Fix AZ capital loss limitation

[MaxGhenis]

Closes #7289

Summary

Arizona income tax now correctly uses loss-limited capital gains instead of raw long-term capital gains when calculating the 25% LTCG subtraction.

Changes

• Updated az_long_term_capital_gains_subtraction to check if net capital gains are positive before applying the subtraction
• When net capital gains are negative (a loss), the subtraction is $0 since no LTCG is "included in federal AGI"
• Added test cases for capital loss scenarios

Technical Details

Per ARS 43-1022(22), the subtraction is for "net long-term capital gain included in federal adjusted gross income". When there's an overall capital loss (STCG losses exceed LTCG), no long-term capital gains make it into federal AGI, so the subtraction should be zero.

Before fix: Person with -$100K STCG and +$50K LTCG would get a $12,500 subtraction (25% of $50K)
After fix: Same person gets $0 subtraction because net capital gains = -$50K (loss)

Test

• Added integration tests for net capital loss and net capital gain scenarios
• All 139 AZ income tax tests pass

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (66ccff2) to head (f990c93).
⚠️ Report is 439 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff              @@
##             main     #7299       +/-   ##
============================================
+ Coverage   72.72%   100.00%   +27.27%     
============================================
  Files        3757         1     -3756     
  Lines       54115        15    -54100     
  Branches      267         0      -267     
============================================
- Hits        39353        15    -39338     
+ Misses      14742         0    -14742     
+ Partials       20         0       -20     

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

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