Add North Dakota renter's refund

[daphnehanse11]

Summary

• Adds North Dakota's Renter's Refund for age-65-or-permanently-and-totally-disabled renters.
• Models the renter refund formula from Tax Commissioner guidance: 20% of eligible annual rent less 4% of income, with a $5 minimum qualifying refund and a $600 cap.
• Adds focused eligibility and refund amount tests, including the state example, cap, ineligible filer, and minimum-refund edge case.

Closes #8199.

Homeowner follow-up: #8325 tracks the separate North Dakota Homestead Property Tax Credit model.

Value Review

• The Tax Commissioner page sets eligibility at age 65 or older, or permanent and total disability.
• The Tax Commissioner page limits renter income to $70,000, including spouse and dependent income for the calendar year preceding the assessment date, and says renters have no asset limitation.
• The Tax Commissioner page says renters are eligible if 20% of annual rent exceeds 4% of annual income, with refunds up to $600.
• The Tax Commissioner guideline says qualifying applicant refunds may not exceed $600 and will be at least $5.

Modeling Notes

• This PR models the renter refund only. The homeowner Homestead Property Tax Credit reduces taxable value rather than issuing a direct renter-style refund, and is tracked in Model North Dakota Homestead Property Tax Credit #8325.
• The model uses adjusted_gross_income as the closest available proxy for the program's income-from-all-sources measure after eligible medical expense deductions.
• The model uses rent as eligible rent, assuming it excludes utilities, furniture, garage rent, federal rent subsidy, and rent paid for property-tax-exempt quarters.
• The model uses is_disabled as the closest available proxy for North Dakota's permanent-and-total disability requirement.

Sources:

• North Dakota Office of State Tax Commissioner, Renter's Refund: https://www.tax.nd.gov/renters-refund
• North Dakota Office of State Tax Commissioner, Homestead Property Tax Credit and Renter's Refund: https://www.tax.nd.gov/homestead-property-tax-credit-and-renters-refund
• North Dakota Office of State Tax Commissioner, Property Tax Credits for North Dakota Homeowners and Renters: https://www.tax.nd.gov/sites/www/files/documents/guidelines/homestead-veterans-renters/credits-for-nd-homeowners-renters-guideline.pdf#page=7

Tests

• uv run policyengine-core test policyengine_us/tests/policy/baseline/gov/states/nd/tax/property/renters_refund -c policyengine_us
• uv run pytest policyengine_us/tests/test_parameter_files.py policyengine_us/tests/test_system_import.py -q
• uv run --extra dev ruff check policyengine_us/variables/gov/states/nd/tax/property/renters_refund

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (8cb60e7) to head (21e26a3).
⚠️ Report is 23 commits behind head on main.

Additional details and impacted files

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  Coverage   100.00%   100.00%           
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  Files            3         2    -1     
  Lines           63        31   -32     
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- Hits            63        31   -32     

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

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