fix(typescript-input): dedupe object type aliases referenced by properties

[schani]

The bug

TypeScript source used as quicktype's input, where an object type alias
(type X = { ... }) is referenced by a property, produced a duplicate
type. For example:

type AuthTokens = {
    accessToken: string;
    refreshToken: string;
    idToken: string;
};

class UserInfo {
    username: string;
    password?: string;
    tokens?: AuthTokens;
}

node dist/index.js --src-lang typescript --lang typescript input.ts produced
both an AuthTokens interface and a spurious duplicate Tokens interface,
with UserInfo.tokens typed Tokens instead of AuthTokens. This did not
happen when AuthTokens was a class or interface, matching the original
report.

Root cause

quicktype-typescript-input builds a JSON Schema from TypeScript source via
the pinned typescript-json-schema package. That package's getTypeDefinition
only emits a $ref to a named definition when the referenced symbol is
ref-able; for type X = {...} aliases, the resolved property type is an
"anonymous object type" internally (as opposed to a class/interface,
which is not anonymous), so it gets fully inlined instead of $ref'd unless
the generator's aliasRef option is enabled — and enabling that option
introduces its own indirection artifacts (extra __type/_1-suffixed
definitions, chained refs) rather than a clean fix.

The fix

packages/quicktype-typescript-input/src/index.ts already monkey-patches the
typescript-json-schema generator's getTypeDefinition in one place
(patchGeneratorForBuiltinTypes, for Map/unsupported builtins). This PR
adds a second, analogous patch, patchGeneratorForTypeAliases: when a
property's type resolves to an anonymous object type that is directly backed
by a (non-generic) top-level type-alias symbol, it rewrites the definition
into a $ref pointing at that alias's own top-level schema definition
instead of letting typescript-json-schema inline a structural duplicate.

Test coverage

Added to test/unit/typescript-input.test.ts (the existing unit-test file
for schemaForTypeScriptSources):

• A regression test reproducing the exact issue Duplicate types from TypeScript input #879 scenario, asserting the
  schema has exactly AuthTokens and UserInfo definitions (no duplicate),
  and that UserInfo.tokens is a $ref to AuthTokens.
• A companion test confirming the pre-existing, already-correct behavior for
  class-based property types is unchanged.

Note: quicktype's fixture-test framework (test/inputs/json,
test/inputs/schema, test/languages.ts, test/fixtures.ts) only drives
quicktype's output side — TypeScript is registered there solely as an
output language, never as an input source — so there is no fixture mechanism
for "TypeScript source as input." schemaForTypeScriptSources already has
direct unit-test coverage for this reason, which is where this regression
test was added.

Verification

• npm run build passes.
• npm run test:unit — all 26 files / 172 tests pass (was 12 tests in the
  TypeScript-input file, now includes the 2 new ones).
• Manually re-ran the exact issue repro
  (node dist/index.js --src-lang typescript --lang typescript input.ts)
  before and after: before, output has both AuthTokens and duplicate
  Tokens; after, only AuthTokens is emitted and UserInfo.tokens: AuthTokens.
• Also spot-checked interface, array-of-alias, and generic-alias shapes to
  confirm no regressions in already-working cases.

CI will additionally run the full fixture suite across all output languages.

Fixes #879

🤖 Generated with Claude Code

[github-actions]

No generated-output differences

✅ Generated outputs are unchanged between the PR base and head revisions.

[github-actions]

No generated-output differences

✅ Generated outputs are unchanged between the PR base and head revisions.