fix(typescript-zod): preserve null for nullable date-time fields

[schani]

Bug

The TypeScript-Zod renderer emitted nullable date-time fields as z.union([z.coerce.date(), z.null()]). Zod tries union members in order, and z.coerce.date() happily coerces null into new Date(null) (the Unix epoch, 1970-01-01T00:00:00.000Z) instead of failing, so the z.null() branch was never reached. A JSON field with createdAt: null round-tripped as an epoch timestamp instead of staying null.

Root cause

In packages/quicktype-core/src/language/TypeScriptZod/TypeScriptZodRenderer.ts, the union-type case built the z.union([...]) member list in whatever order unionType.getChildren() returned them, with no regard for the fact that Zod's coercing schemas (like z.coerce.date()) will happily accept null and coerce it, silently shadowing a z.null() member placed after it.

Fix

Sort the union's children so z.null() is emitted first in the generated z.union([...]) call. Verified directly against the installed zod package:

z.union([z.coerce.date(), z.null()]).parse(null) // => 1970-01-01T00:00:00.000Z (WRONG, pre-fix)
z.union([z.null(), z.coerce.date()]).parse(null) // => null (correct, post-fix)
z.union([z.null(), z.coerce.date()]).parse("2023-01-15T10:30:00Z") // => Date object (still works)

Test coverage

test/inputs/json/priority/bug2590.json (added in #2879 for an unrelated Dart bug) already contains an object with a nullable date-time field, including an array element where it's null. It was previously skipped for typescript-zod in test/languages.ts with a comment pointing at this exact issue (// z.coerce.date() coerces null to the Unix epoch: #2880). This PR removes that skip entry so the fixture now runs for typescript-zod, exercising this regression end-to-end (generate → compile → round-trip via zod's parse/serialize).

Verification

• Reproduced the bug before the fix: node dist/index.js -l typescript-zod sample.json on bug2590.json emitted z.union([z.coerce.date(), z.null()]); confirmed at the zod runtime level that .parse(null) returned an epoch Date.
• Implemented the fix, rebuilt (npm run build passes), and confirmed the generated code now emits z.union([z.null(), z.coerce.date()]), with .parse(null) correctly returning null and normal date-time strings still parsing to Date objects.
• Ran the full typescript-zod fixture suite locally: QUICKTEST=true FIXTURE=typescript-zod script/test — all 45/45 tests passed, including bug2590.json.

Fixes #2880

🤖 Generated with Claude Code