Avoid TANF calculation for non-students in SNAP student eligibility

[anth-volk]

SNAP student ineligibility currently evaluates the TANF exception by requesting tanf_person even when the person is not a higher-education student.

In a traced single-household calculation for a non-college adult, is_snap_ineligible_student returned False but still requested tanf_person once. Because tanf_person can enter the broader TANF calculation tree, this unnecessarily pulls state TANF and county-related work into households that cannot be SNAP-ineligible students in the first place.

Expected behavior:

• If is_snap_higher_ed_student is false, skip the SNAP student exception tree entirely.
• Preserve existing behavior for actual higher-education students, including the TANF exception.

Suggested implementation:

• Add defined_for = "is_snap_higher_ed_student" to is_snap_ineligible_student.
• Add trace-based regression coverage proving tanf_person and tanf are not requested for non-higher-ed people.

Validation target:

• Existing SNAP student eligibility YAML cases should continue to pass.
• Broader SNAP baseline tests should remain unchanged.