Add North Dakota renter's refund

[daphnehanse11]

Summary

• Adds North Dakota's Renter's Refund for age-65-or-disabled renters.
• Models the rent burden formula from the Tax Commissioner page: 20% of annual rent less 4% of income, capped at .
• Adds focused eligibility and refund amount tests.

Closes #8199.

Note: this PR covers the renter refund side of the issue. The homeowner homestead property tax credit has separate valuation and asset-limit logic and should be modeled separately.

Tests

• /Users/daphnehansell/Documents/GitHub/policyengine-us/.venv/bin/python policyengine_us/tests/test_batched.py policyengine_us/tests/policy/baseline/gov/states/nd/tax/property/renters_refund --batches 1
• /Users/daphnehansell/Documents/GitHub/policyengine-us/.venv/bin/ruff check policyengine_us/variables/gov/states/nd/tax/property/renters_refund
• /Users/daphnehansell/Documents/GitHub/policyengine-us/.venv/bin/python -m pytest policyengine_us/tests/test_parameter_files.py policyengine_us/tests/test_system_import.py

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (8cb60e7) to head (b1dcb08).
⚠️ Report is 23 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main     #8314   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
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  Files            3         2    -1     
  Lines           63        30   -33     
=========================================
- Hits            63        30   -33     

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

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