Add NY tax calculation for filters with AGI above $25 million

[DTrim99]

Fixes #6872

[DTrim99]

Issue Found and Fixed

After comparing our calculation with TaxAct/TaxSim results, we discovered our NY tax calculation was ~$150K too low for filers with AGI above $25 million.

The Problem

For NY AGI > $25M, Worksheet 11 in the IT-201 instructions requires:

Tax = Taxable Income × 10.9% (a flat rate on ALL income)

The supplemental tax code was designed to detect this case using:

agi_limit = single.thresholds[-1]  # Last bracket threshold
# ...
return where(ny_agi > agi_limit, supplemental_tax_high_agi, ...)

However, starting in 2022, NY moved to one less bracket and the parameter file was updated:

- threshold:
    2021-01-01: 25_000_000
    2022-01-01: .inf  # ← Last threshold is now infinity!

Since thresholds[-1] = ∞, the condition ny_agi > agi_limit was never true, so the flat 10.9% recapture was never triggered.

The Fix

1. Added explicit high_agi_threshold parameter set to $25,000,000
2. Updated ny_supplemental_tax.py to use this parameter instead of thresholds[-1]

Results

Before	After	TaxAct
$3,494,121	$3,644,120	$3,644,120

Now matches TaxAct exactly (within rounding).

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.55%. Comparing base (fd6b2c5) to head (abeb093).
⚠️ Report is 47 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff             @@
##              main    #6914      +/-   ##
===========================================
- Coverage   100.00%   95.55%   -4.45%     
===========================================
  Files           11        1      -10     
  Lines          146       45     -101     
  Branches         0        2       +2     
===========================================
- Hits           146       43     -103     
- Misses           0        1       +1     
- Partials         0        1       +1     

Flag	Coverage Δ
unittests	95.55% <100.00%> (-4.45%)	⬇️

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[PavelMakarchuk]

@DTrim99 checks fail

[PavelMakarchuk]

Why do we need a separate function for this? Can we not just do it in the formula? I assume we wont reuse this?

[PavelMakarchuk]

Why not +1 to account for the threshold. Not sure why we need a $100 buffer

[DTrim99]

📋 PR Review - NY Supplemental Tax Fix for High AGI (>5M)

✅ Summary: APPROVE

This PR correctly fixes a critical bug in the NY supplemental tax calculation for high-income filers (AGI > $25 million).

---

🔍 The Issue

For NY AGI > $25M, the tax should be calculated as:

Tax = Taxable Income × 10.9% (flat rate on ALL income)

The previous code used thresholds[-1] to detect this threshold, but starting in 2022, the parameter file changed:

- threshold:
    2021-01-01: 25_000_000
    2022-01-01: .inf  # ← Last threshold is now infinity!

Since thresholds[-1] = ∞, the condition ny_agi > agi_limit was never true, so the flat 10.9% recapture was never triggered.

---

✅ The Fix

Added a helper function get_last_finite_threshold() that returns the last non-infinity threshold:

def get_last_finite_threshold(scale):
    thresholds = scale.thresholds
    if np.isinf(thresholds[-1]):
        return thresholds[-2]  # Returns $25M
    return thresholds[-1]

---

🟢 Validation Results

Check	Result	Notes
CI Status	✅ Passing	All 10 checks pass
TaxAct Validation	✅ Matches	$3,644,120 (exactly matches TaxAct)
Before/After	✅ Fixed	$3,494,121 → $3,644,120 ($150K difference corrected)
Test Coverage	✅ Good	Integration test + 2024 unit test added
Hard-coded values	✅ None	Uses parameter thresholds dynamically

---

✅ Test Cases Added

1. Integration test (integration.yaml): Single filer with $33.4M AGI → $3,644,120 tax
2. Unit test (ny_supplemental_tax.yaml): 2024 single filer with AGI > $25M verifying the flat 10.9% calculation

---

🟡 Minor Suggestions (Optional)

1. Changelog wording: Change "filters" to "filers":

   - Add NY tax calculation for filers with AGI above $25 million integration test.

2. Reference: Consider adding the IT-201 worksheet reference (Worksheet 11) to the variable docstring.

---

🚀 Ready to Merge

All CI checks pass and the fix correctly restores the $25M threshold behavior for 2022+ tax years.

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🤖 Automated review by Claude Code