fix(schema): union allOf properties instead of intersecting them

[schani]

Bug

When a JSON Schema allOf combined a $ref to a closed object schema
(additionalProperties: false) with an inline closed object schema, quicktype
generated a class missing properties — in the reported case, all properties
were dropped.

Example (simplified from the issue):

{
  "Dog": {
    "allOf": [
      { "$ref": "#/definitions/Animal" },
      {
        "type": "object",
        "additionalProperties": false,
        "properties": { "Bar": { "type": ["null", "string"] } }
      }
    ]
  },
  "Animal": {
    "type": "object",
    "additionalProperties": false,
    "required": ["discriminator"],
    "properties": {
      "Foo": { "type": ["null", "string"] },
      "discriminator": { "type": "string" }
    }
  }
}

Animal declares Foo/discriminator; the inline branch declares Bar.
Generating C# from this schema produced:

public partial class Dog
{
}

Foo, discriminator, and Bar were all missing. Removing
additionalProperties: false worked around the bug (all properties then
merged correctly), which pointed at the root cause.

Root cause

IntersectionAccumulator.updateObjectProperties in
packages/quicktype-core/src/rewrites/ResolveIntersections.ts computes the
combined property set for an allOf intersection. For any property name that
appeared in the accumulated set but not in the current branch (or vice versa),
the old code deleted it from the accumulator instead of keeping it — i.e. it
intersected the named property sets across branches, rather than unioning
them. additionalProperties: false is only supposed to control whether names
outside that union are permitted; it should not cause a property declared by
only one allOf branch to be dropped entirely.

Fix

Named properties from every allOf branch are now always unioned into the
result. additionalProperties/closedness is still combined separately (as
before) and continues to determine whether names outside the union are
allowed — so the type produced from all-closed branches is still closed
overall, it just no longer discards properties that only one branch declared.

Test coverage

Added test/inputs/schema/allof-closed-objects.schema, a JSON Schema fixture
reproducing the bug (closed $ref base + closed inline object combined via
allOf), with:

• allof-closed-objects.1.json — a positive sample proving properties from
  both allOf branches (including the required field on the $ref branch)
  round-trip correctly.
• allof-closed-objects.1.fail.json — a negative sample (omits the required
  discriminator field) proving required-property enforcement still works.

This is discovered automatically by every registered JSONSchemaFixture (per
test/fixtures.ts), so it now runs across all schema-input languages in CI.

Verification

• Reproduced the bug on unfixed master using the repro from the issue
  (node dist/index.js --src-lang schema --lang cs container.schema):
  confirmed Dog was generated empty.
• Applied the fix; re-ran the same repro — Dog is now generated with
  Foo, Discriminator, and Bar.
• npm run build passes.
• Ran QUICKTEST=true FIXTURE=schema-typescript script/test locally (72
  fixtures, all passing, including the new allof-closed-objects.schema with
  both its positive and negative samples).
• Other language fixtures (C#, etc.) require toolchains not available in this
  sandbox; CI will validate those.

Fixes #1152

🤖 Generated with Claude Code

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