fix(core): preserve outer oneOf title when flattened with null

[schani]

Bug

A JSON Schema oneOf union of single-value string enums that carries its
own title (e.g. the "enum of enums" shape Rust's schemars emits) loses
that title when the union is made nullable via an enclosing anyOf with
{ "type": "null" }. quicktype instead names the generated type after the
first variant's title.

Repro:

{
  "properties": {
    "format": {
      "anyOf": [{ "$ref": "#/definitions/DataFormat" }, { "type": "null" }]
    }
  },
  "definitions": {
    "DataFormat": {
      "title": "DataFormat",
      "oneOf": [
        { "title": "Turtle", "type": "string", "enum": ["turtle"] },
        { "title": "RDF XML", "type": "string", "enum": ["rdf_xml"] },
        { "title": "N-Triples", "type": "string", "enum": ["n_triples"] },
        { "title": "N-Quads", "type": "string", "enum": ["n_quads"] }
      ]
    }
  }
}

node dist/index.js -s schema schema.json --lang ts --prefer-types -o out.ts

Before:

format: Turtle | null;
export type Turtle = "turtle" | "rdf_xml" | "n_triples" | "n_quads";

After:

format: DataFormat | null;
export type DataFormat = "turtle" | "rdf_xml" | "n_triples" | "n_quads";

Root cause

When the inline DataFormat union is flattened alongside null from the
enclosing anyOf, UnionBuilder's attributesForTypes collapses the
nested homogeneous union into its single non-null member type, but the
title attribute carried by that nested union stayed attached to the
discarded nullable-union node instead of the collapsed-into type. Naming
then fell back to the first variant's own title. This affected every output
language (verified TypeScript and Go), since the defect is in
quicktype-core, not a renderer.

Fix

packages/quicktype-core/src/UnionBuilder.ts: when computing attributes for
types in attributesForTypes, detect nested unions that are homogeneous
(all members share one kind) and sit alongside only null in an enclosing
union — i.e. unions that get fully absorbed into a single non-null type
during unification — and fold their own attributes (title, description,
etc.) onto that resulting type, at increased distance so more specific
names still win where applicable.

Tests

• test/inputs/schema/enum-of-enums-null.schema (+ .1.json/.2.json
  samples): a new end-to-end JSON Schema fixture reproducing the exact
  reported shape, picked up automatically by the schema fixture harness for
  every target language.
• test/unit/enum-of-enums-null-name.test.ts: a focused unit test asserting
  the generated TypeScript names the type DataFormat and never falls back
  to Turtle, since fixture round-tripping alone can't assert on the chosen
  identifier.

Verification

• npm run build passes.
• npm run test:unit — 164/164 passing.
• Repro re-run locally for both --lang ts and --lang go: both now emit
  DataFormat instead of Turtle.
• QUICKTEST=true FIXTURE=schema-golang script/test — 68/68 passing,
  including the new enum-of-enums-null.schema fixture.
• QUICKTEST=true FIXTURE=schema-typescript script/test — passing except a
  single pre-existing, unrelated failure on vega-lite.schema (reproduces
  identically on master without this change; a noUncheckedIndexedAccess-style
  strict-mode TS2411 clash unrelated to unions/enums). The new
  enum-of-enums-null.schema fixture passes.
• CI will run the full fixture matrix across all languages.

Fixes #2410

🤖 Generated with Claude Code

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