Harden ACA ZIP3 parsing

[daphnehanse11]

Partially fixes #8675.

Summary

• Makes three_digit_zip_code parse ZIP codes as text before deriving ZIP3.
• Returns an empty ZIP3 for missing or nonnumeric ZIP codes instead of raising during ACA SLCSP paths.
• Adds tests for UNKNOWN, empty, nonnumeric, ZIP+4, and leading-zero ZIP values.

Tests

• uv run pytest policyengine_us/tests/policy/baseline/household/demographic/geographic/test_three_digit_zip_code.py
• git diff --check

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (f9e58e7) to head (4f67e8d).
⚠️ Report is 161 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main     #8724   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            3         1    -2     
  Lines           55        13   -42     
=========================================
- Hits            55        13   -42     

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

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