fix(schema): keep allOf-merged properties when additionalProperties: false

[schani]

Bug

Issue #1257: when a JSON Schema allOf merges branches that each set
additionalProperties: false (a very common pattern, e.g. one branch is a
$ref to a shared base object and the other is inline), quicktype's
TypeScript output dropped all the properties instead of merging them.

Repro:

{
  "definitions": {
    "AmountAndFrequency": {
      "type": "object",
      "properties": {
        "amount": { "type": "number" },
        "frequency": { "type": "string", "enum": ["Weekly", "Monthly", "Annually"] }
      },
      "additionalProperties": false,
      "required": ["amount", "frequency"]
    },
    "Income": {
      "allOf": [
        { "$ref": "#/definitions/AmountAndFrequency" },
        {
          "type": "object",
          "properties": {
            "type": { "type": "string", "enum": ["GrossSalaryAmount", "Overtime"] },
            "description": { "type": "string" }
          },
          "additionalProperties": false,
          "required": ["type"]
        }
      ]
    }
  },
  "$ref": "#/definitions/Income"
}

node dist/index.js --src schema.json --src-lang schema --lang ts

Before:

export interface Schema {
}

After:

export interface Schema {
    amount:       number;
    frequency:    Frequency;
    description?: string;
    type:         Type;
}

Root cause

IntersectionAccumulator.updateObjectProperties in
packages/quicktype-core/src/rewrites/ResolveIntersections.ts merges the
object types being intersected by an allOf. For each property name seen so
far across branches, if the current branch didn't declare that property and
didn't allow additional properties, the property was deleted from the merged
result — even though some other branch did declare it. Since JSON Schema
input in the wild very commonly sets additionalProperties: false on every
branch, this deleted essentially all disjoint properties, frequently leaving
an empty merged object.

Fix

A property known from any allOf branch is now kept in the merged object
regardless of whether another branch restricts additional properties;
additionalProperties: false still correctly disallows properties that no
branch declares (that behavior is untouched — only the erroneous deletion of
already-known properties was removed).

Test coverage

Added test/inputs/schema/all-of-additional-properties-false.schema
(the repro above) with:

• .1.json: a positive round-trip sample with all four merged properties.
• .1.fail.enum.json: a negative sample with an invalid enum value for
  frequency, a property that's only present at all because the merge now
  works — it guards against a regression of this exact bug rather than
  testing something unrelated.

Verification

• npm run build passes.
• npm run test:unit passes (176 tests).
• QUICKTEST=true FIXTURE=schema-typescript script/test passes (exit 0),
  including the new fixture (both the positive and .fail.enum samples).
• Manually re-ran the repro above before/after the fix as shown.
• The other schema-<language> fixtures were not run locally due to
  time/toolchain constraints; CI will validate them.

Fixes #1257

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