fix: enforce custom totals display text

[FanouZeng-TT]

Problem

The pinned totals.json schema requires display_text when a total uses a custom type, but generated Totals, TotalsCreateRequest, and TotalsUpdateRequest aliases currently accept custom entries without it.

Fix

• extract the schema's simple array-item not.enum / then.required condition
• include that condition in the existing schema-driven totals AfterValidator
• regenerate the three totals aliases
• cover custom, known, and request-variant behavior

Validation

• ./generate_models.sh 2026-04-08
• uv run python -m unittest discover -s tests -p "test_*.py" (79 passed)
• uvx pre-commit run --all-files
• git diff --check