fix: enforce conditional numeric bounds in generated models

[yingzhehu-TT]

Summary

Restore JSON Schema if/then numeric bounds, which
datamodel-code-generator drops, by injecting a model_validator alongside the
existing conditional-required family.

Motivation

total.json constrains amount by type: discount / items_discount must be
exclusiveMaximum: 0, and subtotal / fulfillment / tax / fee must be
minimum: 0. The generated Total carries no validator at all, so:

Total(type="discount", amount=500)   # accepted — a positive discount
Total(type="tax", amount=-999)       # accepted — a negative tax

find_conditional_required already handles a then that adds required fields,
but returns early on a then that narrows a range, so this family was never
covered.

Scope

Mirrors the conditional-required trio (find_ / inject_ / _patch_) so both
read the same way. Two differences worth calling out:

• allOf scoping. These rules are carried as allOf branches, which have no
  sibling properties of their own — the object they constrain is the enclosing
  schema. The scan therefore threads the enclosing property set into each branch.
  A rule is still rejected unless it has a single const/enum discriminator and
  a then containing only numeric-bound keywords; anything else is skipped with a
  warning rather than approximated.
• Request variants are not a warning. total_create_request.json strips every
  property, so the rule names fields the class does not have. That is inapplicable
  rather than malformed, and is skipped silently — otherwise every generation run
  would print four spurious warnings.

type remains an open vocabulary: a value carrying no rule (total, or a vendor
value) is unconstrained, and a None amount is not checked.

Validation

python -m unittest discover -s tests — 77 passed (72 before; 5 new).
ruff check adds no diagnostic outside the D,E501 set this repo ignores in
pre-commit; ruff format clean.

Regeneration against the pinned 2026-04-08 spec touches only total.py and
totals.py. Running postprocess_models.py a second time leaves the output
byte-identical.

Behaviour of the generated model, all verified:

input	before	after
type="discount", amount=500	accepted	rejected
type="items_discount", amount=1	accepted	rejected
type="tax", amount=-999	accepted	rejected
type="fee", amount=-1	accepted	rejected
type="discount", amount=-500	accepted	accepted
type="tax", amount=100	accepted	accepted
type="subtotal", amount=0	accepted	accepted
type="total", amount=-5 (no rule)	accepted	accepted
type="custom_vendor_type", amount=-7	accepted	accepted

js-sdk already enforces the same two rules on TotalResponseSchema, so this
closes a cross-SDK divergence.