fix: enforce conditional total amount bounds

[FanouZeng-TT]

Summary

• Add postprocessing for simple if/then numeric bounds that datamodel-code-generator drops.
• Regenerate Total models so discounts must be negative and subtotal/fulfillment/tax/fee amounts must be non-negative.
• Add codegen tests covering finder behavior, injection idempotency, and generated model validation.

Test plan

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

[damaz91]

Already addressed by #71 - thanks!