Add Minnesota Homestead Credit Refund

[MaxGhenis]

Fixes #8397.

Summary

• add Minnesota's homeowner Homestead Credit Refund as a property tax credit/circuit-breaker variable
• parameterize the 2024 and 2025 M1PR refund worksheet percent-of-income, refund-rate, and maximum-refund schedules
• include the credit in gov.states.household.state_property_tax_credits so it flows into taxsim_state_property_tax_credit / state_property_tax_credit
• model M1PR household income using modeled AGI, nontaxable Social Security, retirement contributions, and age/disability/dependent subtractions

Notes

• This covers the regular homeowner Homestead Credit Refund. The separate special property tax refund is not modeled because it requires prior-year homestead property tax and same-owner January 2 information.
• Co-occupant income and miscellaneous nontaxable additions/subtractions on Form M1PR are documented as not separately modeled.

Tests

• uv run python -m policyengine_core.scripts.policyengine_command test policyengine_us/tests/policy/baseline/gov/states/mn/tax/property/homestead_credit_refund/mn_homestead_credit_refund.yaml -c policyengine_us
• uv run ruff check $(git diff upstream/main...HEAD --name-only | rg '\.py$')
• git diff --check upstream/main...HEAD
• uv run coverage erase && git diff upstream/main...HEAD --name-only | xargs uv run python policyengine_us/tests/run_selective_tests.py --coverage --files && uv run coverage report --fail-under=0 --skip-covered --skip-empty

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (92fdab8) to head (a1a19e0).
⚠️ Report is 58 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main     #8417   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            2         3    +1     
  Lines           57        70   +13     
  Branches         1         0    -1     
=========================================
+ Hits            57        70   +13     

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

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.