Add Tennessee elderly property tax relief

[daphnehanse11]

Summary

• Add Tennessee Property Tax Relief for Elderly Homeowners.
• Parameterize age, residential assessment rate, income limits, and qualifying property value cap.
• Include 2025 and 2026 values: income limit 37,530 in 2025 and 38,470 in 2026; property value cap 32,700 for both years.
• Add focused eligibility and amount tests, and include the relief in state property tax credits from 2025.

Modeling notes

• Uses real_estate_taxes to preserve local tax-rate differences.
• Converts the published full-market-value cap to an assessed-value cap using Tennessee's 25% residential assessment rate.
• Uses adjusted gross income as the modeled tax-unit income proxy.

Tests

• uv run python policyengine_us/tests/test_batched.py policyengine_us/tests/policy/baseline/gov/states/tn/tax/property/elderly_property_tax_relief --batches 1
• .venv/bin/python -m pytest policyengine_us/tests/test_parameter_files.py policyengine_us/tests/test_system_import.py policyengine_us/tests/code_health/parameters.py policyengine_us/tests/code_health/variable_names.py
• /Users/daphnehansell/Documents/GitHub/policyengine-us/.venv/bin/ruff format --check .
• /Users/daphnehansell/Documents/GitHub/policyengine-us/.venv/bin/ruff check .

Closes #8203

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (53069e0) to head (771f837).
⚠️ Report is 7 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main     #8307   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            1         4    +3     
  Lines           21        43   +22     
=========================================
+ Hits            21        43   +22     

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.

[MaxGhenis]

Converted this to draft for the same data-readiness reason as #8303, but scoped to property tax data. This PR relies on assessed_property_value, which is currently only a PolicyEngine-US input variable and is not present/imputed in policyengine-us-data; rent and real_estate_taxes are data-backed, but assessed value is not. The rule tests are still useful, but baseline wiring into refundable/property-tax-credit aggregates should wait until we either add an assessed-value data/imputation path or revise the formula to rely only on data-backed inputs.