fix(schema): emit min/max constraints for applicable type kinds

[schani]

Bug

Reported in #2557: converting a JSON Schema through quicktype's
--src-lang schema ... --lang schema round trip silently dropped
minLength/maxLength (on strings) and minimum/maximum (on
numbers/integers) from the output schema, while pattern and
description survived correctly. Since the schema-to-schema pass is the
first stage of every other output language too, this also meant the
constraints never reached downstream generators (TypeScript, zod, etc.).

Root cause

MinMaxConstraintTypeAttributeKind.addToSchema in
packages/quicktype-core/src/attributes/Constraints.ts had an inverted
applicability guard:

if (this._typeKinds.has(t.kind)) return;

_typeKinds holds the type kinds a given constraint kind actually
applies to ({"integer", "double"} for min/max value, {"string"} for
min/max length). As written, the method bailed out and skipped emitting
the constraint into the schema exactly when the type's kind was
applicable — the opposite of the intended behavior. Compare with
PatternTypeAttributeKind.addToSchema, which has the correct polarity
(if (t.kind !== "string") return;) and is why pattern round-tripped
fine while minLength/maxLength/minimum/maximum did not.

Fix

Inverted the guard:

if (!this._typeKinds.has(t.kind)) return;

This is a one-line change; the zod/TypeScript renderers not emitting
these constraints into generated validation code is a separate, unrelated
missing feature and is out of scope for this PR.

Test coverage

• Added a new schema-schema fixture (test/fixtures/schema/main.js)
  that validates the round-tripped JSON Schema output against the
  original sample using AJV, registered in test/languages.ts /
  test/fixtures.ts.
• Added test/inputs/schema/schema-constraints.schema with:
  • schema-constraints.1.json — a sample that satisfies
    minLength/maxLength/minimum/maximum.
  • schema-constraints.{1,2}.fail.minmaxlength.json — samples that
    violate minLength/maxLength and must be rejected by the
    generated schema.
  • schema-constraints.{3,4}.fail.minmax.json — samples that violate
    minimum/maximum and must be rejected by the generated schema.

These fail samples only pass once the generated schema actually contains
the constraints, so they reproduce the bug on unfixed code and verify the
fix.

Verification (local)

• npm run build — passes.
• QUICKTEST=true FIXTURE=schema-schema script/test — 68/68 passed
  (including the new schema-constraints.schema cases).
• npm run test:unit — 163/163 tests passed across 25 files.
• Manually re-ran the reporter's repro
  (node dist/index.js --src-lang schema bad-schema.json --lang schema)
  and confirmed minLength/maxLength/minimum/maximum are now
  present in the generated schema.
• Full multi-language fixture suite (FIXTURE=schema, all
  schema-<lang> targets) is left to CI, since several target-language
  toolchains (e.g. dotnet) aren't available in this environment.

Fixes #2557

🤖 Generated with Claude Code