Fix NJ gross income formula that breaks API reform calculations

[PavelMakarchuk]

Summary

• Replace ParameterNode iteration (for cat in cats) with explicit category references in nj_gross_income formula
• The iteration pattern is incompatible with the API's clone()-based reform path, causing all NJ tax variables to silently return null under any reform
• NJ was the only state affected — all other 50 states + DC work correctly

Fixes #7742

Test plan

• All 318 NJ tests pass
• Verify NJ reform calculations work on the API after deployment

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (752fae4) to head (fbe4d2a).
⚠️ Report is 31 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main     #7743   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            2         1    -1     
  Lines           13        18    +5     
=========================================
+ Hits            13        18    +5     

Flag	Coverage Δ
unittests	100.00% <100.00%> (ø)

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[MaxGhenis]

Reviewed the fix and searched for similar patterns across the codebase.

Fix is correct — replacing ParameterNode directory iteration with explicit attribute access is more robust and unblocks NJ reform calculations on the API.

No other instances need fixing — checked all 12 cases of parameter iteration in the codebase. NJ loss_eligible_categories was the only directory-based ParameterNode being iterated. All other 11 instances (itemized_deductions, gross_income_sources, deductions.sources, etc.) iterate over unit: list parameter files that return plain Python lists, unaffected by clone().

Regression test not feasible — the bug can't be reproduced locally (even with clone() + parameter.update()). It's specific to the deployed API environment, compounded by silent exception swallowing (API issue #3322).