Add US government bond interest exemption to AL, AR, CA, DC, DE state income taxes (+ tests for AZ, CO, CT, GA, HI)

[MaxGhenis]

Summary

Adds the US government bond interest exemption (us_govt_interest subtraction) for states that were missing it, and adds test coverage for all 10 states that now have this exemption implemented.

Changes by state

Code changes (added exemption to state AGI calculations):

• Alabama (AL)
• Arkansas (AR)
• California (CA)
• District of Columbia (DC)
• Delaware (DE)

Test coverage (YAML test cases confirming exemption is included):

• Arizona (AZ) — already had implementation
• Colorado (CO) — already had implementation
• Connecticut (CT) — already had implementation
• Georgia (GA) — already had implementation
• Hawaii (HI) — already had implementation

Test plan

Each state now has YAML test coverage in policyengine_us/tests/policy/baseline/gov/states/{STATE}/tax/income/{STATE}_us_govt_bond_interest.yaml that verifies:

• The US government bond interest is correctly identified as a subtraction from AGI
• The parameter is properly integrated into the state's income tax calculation

Run: uv run pytest policyengine_us/tests/ -k "us_govt_bond_interest" to validate all tests pass.

Closes

Closes #7619 (AL), #7620 (AR), #7621 (AZ), #7622 (CA), #7623 (CO), #7624 (CT), #7625 (DC), #7626 (DE), #7627 (GA), #7628 (HI)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 77.54%. Comparing base (55a0045) to head (bc15c5b).
⚠️ Report is 37 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #7667       +/-   ##
===========================================
- Coverage   97.82%   77.54%   -20.28%     
===========================================
  Files           3     3904     +3901     
  Lines          92    56341    +56249     
  Branches        2      271      +269     
===========================================
+ Hits           90    43692    +43602     
- Misses          1    12609    +12608     
- Partials        1       40       +39     

Flag	Coverage Δ
unittests	77.54% <ø> (-20.28%)	⬇️

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