fix(zod): fall back to z.union for discriminated unions with empty-object members

[pullfrog]

Closes #3914.

Problem

When an OpenAPI oneOf/anyOf with a discriminator referenced an empty object schema (type: object with no properties), the generated Zod schema failed to type-check:

export const zEmpty = z.record(z.string(), z.unknown());

export const zTestResponse = z.discriminatedUnion('type', [
    zEmpty.extend({ type: z.literal('Empty') }), // ❌ `.extend` does not exist on z.record
    zWithValue.extend({ type: z.literal('WithValue') })
]);

zEmpty is rendered as z.record(...) (because the empty object has no properties but the parser injects additionalProperties: { type: 'unknown' }), and z.record(...) has no .extend(...) method.

Fix

tryBuildDiscriminatedUnion in packages/openapi-ts/src/plugins/zod/shared/discriminated-union.ts now resolves each $ref member and returns null when the resolved schema is record-shaped (type: 'object', no properties, additionalProperties set). This delegates to the existing z.union(...) fallback path, which renders compileable z.object({ type: z.literal(...) }).and(zEmpty) members for v3/v4 and z.intersection(...) for mini.

export const zTestResponse = z.union([
    z.object({
        type: z.literal('Empty')
    }).and(zEmpty),
    z.object({
        type: z.literal('WithValue')
    }).and(zWithValue)
]);

Tests

Added discriminator-empty-object-member scenario to all four zod test files (v3/v4 × 3.0.x/3.1.x) with new specs/{3.0.x,3.1.x}/discriminator-empty-object-member.yaml inputs and 12 generated snapshots (3 dialects × 2 OAS versions × 2 zod packages).

• pnpm vitest run --project @test/openapi-ts-zod-v3 --project @test/openapi-ts-zod-v4 — 210 passed
• pnpm ty @hey-api/openapi-ts — clean
• pnpm lint — no new warnings

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🦋 Changeset detected

Latest commit: 8fa9f3a

The changes in this PR will be included in the next version bump.

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Name	Type
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commit: 8fa9f3a

[codecov]

Codecov Report

❌ Patch coverage is 0% with 7 lines in your changes missing coverage. Please review.
✅ Project coverage is 37.69%. Comparing base (0c865e1) to head (8fa9f3a).

Files with missing lines	Patch %	Lines
...i-ts/src/plugins/zod/shared/discriminated-union.ts	0.00%	3 Missing and 4 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3915      +/-   ##
==========================================
- Coverage   37.70%   37.69%   -0.02%     
==========================================
  Files         582      582              
  Lines       20844    20851       +7     
  Branches     6063     6067       +4     
==========================================
  Hits         7860     7860              
- Misses      10570    10573       +3     
- Partials     2414     2418       +4     

Flag	Coverage Δ
unittests	37.69% <0.00%> (-0.02%)	⬇️

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