fix(json-schema): stop merged type descriptions overriding property descriptions

[schani]

Bug

Converting a Kubernetes CRD's OpenAPI v3 schema through quicktype's
schema-to-schema mode (--lang schema --src-lang schema) produced a
description for definitions.Allow.properties.labels that was a garbled
concatenation of eleven unrelated description strings from all over the
input schema, instead of the single correct description that actually
belongs to that property ("The labels to apply to the policy").

Root cause

quicktype's type unifier merges structurally identical types across the
whole schema graph — e.g. every plain {additionalProperties: string}
"map of string" property anywhere in the schema (regardless of which
parent object it lives under) gets unified into a single shared Type
node, since they're structurally indistinguishable.

DescriptionTypeAttributeKind.combine() in
packages/quicktype-core/src/attributes/Description.ts set-unions the
description type attribute from every occurrence when such types get
unified, so that one shared map type ends up carrying descriptions from
every unrelated property that happens to have the same map-of-string
shape.

JSONSchemaRenderer.definitionForObject() in
packages/quicktype-core/src/language/JSONSchema/JSONSchemaRenderer.ts
built each property's schema via schemaForType(p.type). For a type with
no $ref (i.e. it gets inlined directly), schemaForType already stamps
the merged type-level description onto the inlined schema. The renderer
then only applied the correct, property-specific description
(descriptionForClassProperty, which is not merged across unrelated
properties) when prop.description was still undefined — so the wrong,
merged description always won whenever it was present.

Fix

Always prefer the property-specific description over the (possibly
merged) type-level one when rendering an object's properties, falling
back to the type-level description only when no property-specific
description exists. This is a small, localized change in
JSONSchemaRenderer.definitionForObject.

Test coverage

Added a description-unification.schema input plus a dedicated
schema-schema fixture (JSONSchemaToSchemaFixture in
test/fixtures.ts) that runs quicktype schema-to-schema and asserts on
the description text of the generated output for two unrelated
properties that share the same map-of-string shape. The existing
JSONSchemaFixture/JSONSchemaJSONFixture classes only validate/compile
generated code and don't inspect JSON Schema description text, so this
required new (minimal) fixture plumbing rather than reusing an existing
one. Also wired the new schema-schema fixture into
.github/workflows/test-pr.yaml.

Verification (local)

• Confirmed the new fixture fails on unfixed code (reverted just the
  renderer fix and re-ran QUICKTEST=true FIXTURE=schema-schema script/test; it failed with the merged description) and passes with
  the fix restored.
• Re-ran the original reporter repro (CRD from the linked gist):
  definitions.Allow.properties.labels.description now correctly reads
  "The labels to apply to the policy" instead of the 11
  newline-joined descriptions.
• npm run build passes.
• npx vitest run test/unit — 176 tests passed, no regressions.
• QUICKTEST=true FIXTURE=schema-typescript script/test — all 72 schema
  fixtures pass, confirming the new schema input compiles cleanly across
  the existing per-language schema fixtures too.
• Full multi-language fixture matrix (CI) is left to CI, given its
  runtime.

Fixes #2801

🤖 Generated with Claude Code

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