fix(typescript): preserve anyOf unions of object types instead of merging

[schani]

Bug

With JSON Schema and the TypeScript target, anyOf of two plain object
types did not turn into a TypeScript union. Instead the two object types
were merged/flattened into a single interface containing all properties
from both, and the referencing property was typed as that merged
interface instead of a union.

Repro schema:

{
  "$ref": "#/definitions/TestObject",
  "definitions": {
    "TestObject": {
      "type": "object",
      "properties": {
        "firstProperty": {
          "anyOf": [{ "$ref": "#/definitions/MyObjectA" }, { "$ref": "#/definitions/MyObjectB" }]
        }
      }
    },
    "MyObjectA": { "type": "object", "properties": { "a": {"type":"string"}, "b": {"type":"number"}, "c": {"type":"boolean"} } },
    "MyObjectB": { "type": "object", "properties": { "d": {"type":"string"}, "e": {"type":"number"}, "f": {"type":"boolean"} } }
  }
}

node dist/index.js --lang typescript --src-lang schema test.schema produced:

export interface Test {
    firstProperty?: MyObject; // wrong: merged interface with a-f
}
export interface MyObject { a?: string; b?: number; c?: boolean; d?: string; e?: number; f?: boolean; }

instead of the expected

export interface Test {
    firstProperty?: MyObjectA | MyObjectB;
}

Root cause

flattenUnions (packages/quicktype-core/src/rewrites/FlattenUnions.ts)
always collapsed non-canonical unions whose members were class/object
types via UnifyUnionBuilder/UnifyClasses, regardless of whether the
target language could actually represent a union of named object types.
TypeScript can (and does, for e.g. array | object anyOf combinations),
but the class/object-union case was always flattened.

Fix

• Added TargetLanguage.supportsUnionsWithMultipleObjectTypes
  (packages/quicktype-core/src/TargetLanguage.ts), defaulting to
  false, and enabled it for TypeScript
  (packages/quicktype-core/src/language/TypeScriptFlow/language.ts).
• flattenUnions now skips flattening a non-canonical union when all its
  members are class/object types and the target language supports
  unions of object types — except when that union still participates in
  an IntersectionType, since IntersectionAccumulator can only
  represent one member per kind and still needs the union flattened
  there.
• Threaded the new flag through Run.ts to flattenUnions.

This only changes behavior for target languages that opt in
(TypeScript); all other languages keep flattening class unions exactly
as before.

Test coverage

Added a new JSON Schema fixture:

• test/inputs/schema/anyof-object-union.schema — the anyOf-of-two-
  objects scenario from the issue (plus an array/object anyOf, which
  already worked, to guard against regressing that case).
• test/inputs/schema/anyof-object-union.1.json — a passing round-trip
  sample.
• test/inputs/schema/anyof-object-union.1.fail.class-union.json — a
  sample mixing properties from both object types, which must be
  rejected now that the two objects are validated as a real union
  (additionalProperties: false on both).

Wired up via a new "class-union" fixture feature enabled for the
typescript language in test/languages.ts, so the .fail.class-union
sample is exercised for schema-typescript.

Verification (local)

• npm run build passes
• QUICKTEST=true FIXTURE=schema-typescript script/test: 71/71 passed,
  including the new fixture and its fail sample
• QUICKTEST=true FIXTURE=schema-python script/test: 71/71 passed
  (confirms non-opted-in languages are unaffected)
• QUICKTEST=true FIXTURE=schema-golang script/test: 71/71 passed
• npm run test:unit: 170/170 passed
• Direct repro now emits firstProperty?: MyObjectA | MyObjectB;
  instead of a merged MyObject interface

CI will run the full fixture/unit suite across all languages.

Fixes #1338

🤖 Generated with Claude Code

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[github-actions]

Generated-output differences

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5957 changed lines — +5931 / −26

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