Skip SNAP student TANF checks for non-students

[anth-volk]

Fixes #8762

Summary

• Add defined_for = "is_snap_higher_ed_student" to is_snap_ineligible_student.
• Add a trace regression test proving non-higher-ed people do not request tanf_person or tanf through SNAP student ineligibility.
• Add a changelog fragment.

Runtime impact

Before this change, a traced single-household calculation for a non-college adult returned False for is_snap_ineligible_student but still requested tanf_person once. That can pull the broader TANF tree, including county-related work, into households where the SNAP student ineligibility rule cannot apply.

After this change, is_snap_ineligible_student is only calculated for people where is_snap_higher_ed_student is true. The new trace test asserts zero tanf_person and tanf requests for a non-higher-ed person. Existing behavior for higher-ed students, including the TANF exception, is preserved.

Tests

• ./.venv/bin/python -m pytest policyengine_us/tests/core/test_snap_student_tanf_dependency.py
• ./.venv/bin/python -m policyengine_core.scripts.policyengine_command test policyengine_us/tests/policy/baseline/gov/usda/snap/eligibility/student/is_snap_ineligible_student.yaml -c policyengine_us
• make format
• Broader SNAP baseline run reported 315 passed, 1 warning; the command wrapper stayed attached after the pass summary, so it was interrupted after the success output.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (194d129) to head (09f823f).
⚠️ Report is 8 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##              main     #8763    +/-   ##
==========================================
  Coverage   100.00%   100.00%            
==========================================
  Files           11         1    -10     
  Lines          171        20   -151     
==========================================
- Hits           171        20   -151     

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

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