Add Idaho senior deduction conformity

[PavelMakarchuk]

Summary

Add Idaho conformity to the federal enhanced senior deduction under HB 559.

What this does

• adds an Idaho subtraction variable that mirrors the federal additional senior deduction
• wires that subtraction into Idaho AGI subtractions starting in 2025
• adds direct and integration regression coverage for the Idaho senior deduction path

Notes

The issue body example had an inconsistent expected Idaho taxable income value. With 200,000 dollars of AGI, 60,000 dollars of itemized deductions, and a 9,000 dollar senior deduction, the coherent Idaho taxable income is 131,000, and that is what the new integration test asserts.

Testing

• python policyengine_us/tests/test_batched.py policyengine_us/tests/policy/baseline/gov/states/id/tax/income/subtractions/id_additional_senior_deduction.yaml --batches 1
• python policyengine_us/tests/test_batched.py policyengine_us/tests/policy/baseline/gov/states/id/tax/income/integration.yaml --batches 1
• python policyengine_us/tests/test_batched.py policyengine_us/tests/policy/baseline/gov/states/id/tax/income --batches 1

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (e8a542a) to head (b2fae50).
⚠️ Report is 28 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main     #7964   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            1         4    +3     
  Lines           17        49   +32     
=========================================
+ Hits            17        49   +32     

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

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