fix(schema): support boolean "false" subschemas

[schani]

Bug

Fixes #1896.

quicktype crashed with Error: Schema "false" is not supported at <ref> on
valid draft-07+ JSON Schema documents that use a boolean subschema
(false) as a property/items value — e.g. "patches": false to mean "this
property is disallowed here." This is a normal part of the JSON Schema spec
(commonly emitted by tools that compile oneOf/allOf branches), and it
appears in the real-world Amazon Selling Partner Listings Feed schema
attached to the issue.

Repro before the fix:

node dist/index.js --src-lang schema --lang csharp amazon.schema
# Error: Schema "false" is not supported at .../properties/messages/items/oneOf/0/properties/patches.

Exit code 1, no code generated at all, for any target language.

Root cause

In packages/quicktype-core/src/input/JSONSchemaInput.ts, toType()
handled schema === true (mapping it to the any primitive) but asserted
against schema === false, throwing SchemaFalseNotSupported and aborting
the whole run. items: false hit a similar rejection in the array-items
handling.

Fix

• false subschemas now map to quicktype's existing none primitive type —
  the same type already used to represent an empty union. A later pass
  (noneToAny, already run in the pipeline) converts none to any before
  rendering, so false now behaves like an always-empty/unknown value
  instead of crashing. true keeps mapping to any as before.
• items: false is now accepted the same way object properties are (was
  previously rejected as "must be string or array").
• resolveIntersections (used for allOf) now treats none as an
  absorbing member, so allOf: [false, ...] correctly resolves to none
  (nothing valid) rather than mis-simplifying.

Test coverage

Added test/inputs/schema/boolean-subschema.schema (+
boolean-subschema.1.json sample), covering:

• false as a plain property value
• items: false inside an array schema
• false inside an allOf (intersection)

This is a new JSON Schema fixture input under test/inputs/schema/, so it
runs automatically against every language's schema-<language> fixture in
CI (per the repo's fixture-based testing convention).

Verification performed locally

• npm run build passes.
• Re-ran the original issue repro (amazon.schema from Can not create C# code or any other from Amazon json schemas #1896) with
  --lang csharp: now generates valid C# (458 lines, exit 0) instead of
  erroring.
• QUICKTEST=true FIXTURE=schema-typescript script/test test/inputs/schema/boolean-subschema.schema: passes.
• QUICKTEST=true FIXTURE=schema-typescript script/test (full TS schema
  suite): passes, aside from one pre-existing, unrelated failure
  (vega-lite.schema hits a TypeScript TS2411 index-signature error) that
  I confirmed also reproduces identically on master without this change.
• npm run test:unit: all 163 unit tests pass.
• The C#-specific fixture (schema-csharp) could not be run locally
  (dotnet is not installed in this environment); CI will validate it and
  the other language fixtures.

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