fix(json-schema): recognize $defs when deriving type names

[schani]

Bug

Fixes #2778.

A JSON Schema that defines a type under $defs and references it got a
different, truncated name in generated output than the name given in the
schema. For example, a type named LightParams under $defs was emitted
as Light in generated TypeScript. Renaming $defs to the legacy
definitions keyword in the same schema produced the correct name,
which pointed at the root cause.

Root cause

Ref.definitionName in packages/quicktype-core/src/input/JSONSchemaInput.ts
only recognized the legacy "definitions" JSON Schema keyword, not "$defs"
(the standard keyword for local definitions since draft 2019-09, used by the
2020-12 draft referenced in the issue). Because of this, a type's $defs key
was never treated as a "given name" for the type. The type kept only its
inferred names (e.g. the referencing property name, plus a name derived
from the input file name), and combineNames in
packages/quicktype-core/src/attributes/TypeNames.ts merged those candidates
by common prefix — which produced the truncated/wrong name.

Fix

definitionName now also recognizes "$defs" alongside "definitions":

if (keyOrIndex(pe) === "definitions" || keyOrIndex(pe) === "$defs")
    return keyOrIndex(defined(arrayLast(this.path)));

Test coverage

• New JSON Schema fixture test/inputs/schema/light.schema +
  test/inputs/schema/light.1.json, reproducing the schema from the issue,
  runs end-to-end (round-trip) for every schema-input language fixture.
• New unit test test/unit/json-schema-definitions.test.ts asserts the
  generated TypeScript interface is named LightParams, not Light. This
  is a unit test rather than a fixture-only check because fixture tests
  validate round-tripping, not the exact generated symbol name — the actual
  regression here.
• Checked test/inputs/schema/*.schema for any other existing input using
  $defs; light.schema is the only one, so no other fixture snapshots are
  affected by this change.

Verification

• npm run build — passes.
• npm run test:unit — 164/164 tests pass (26 files).
• Re-ran the exact repro from the issue:

  node dist/index.js --src-lang schema --lang ts --just-types --no-date-times --alphabetize-properties light.schema.json
  

  Before: export interface Light { ... } (referenced as Light).
  After: export interface LightParams { ... } (referenced as LightParams), matching the reporter's expectation.
• QUICKTEST=true FIXTURE=schema-typescript script/test test/inputs/schema/light.schema — passes.
• biome check on changed files — clean.
• Ran the full schema-typescript fixture set locally; confirmed one pre-existing, unrelated failure (vega-lite.schema hits pre-existing TypeScript compile errors, TS2411, on unmodified master as well — unrelated to this change). CI will validate the rest of the language matrix.

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