Fix CHIP FCEP immigration handling

[daphnehanse11]

Fixes #8530.

Summary

• Removes the undocumented-parent exclusion from CHIP FCEP proxy eligibility.
• Updates FCEP labeling/documentation to reference the From-Conception-to-End-of-Pregnancy option.
• Updates the regression test so undocumented-parent and citizen-parent cases are treated consistently for FCEP.

Tests

• uv run python -m policyengine_core.scripts.policyengine_command test policyengine_us/tests/policy/baseline/gov/hhs/chip/is_chip_eligible_fcep.yaml -c policyengine_us
• git diff --check

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (b5dd136) to head (c20b1ba).
⚠️ Report is 17 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main     #8720   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            3         1    -2     
  Lines           34        18   -16     
=========================================
- Hits            34        18   -16     

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

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