fix(zod): preserve null type when .nullish() is applied to named schema reference

[daniel-rose]

Description

When applying .nullish() or .nullable() to a field that references a named schema (producing a $ref), the generator wrapped the reference in allOf: [{ $ref }] but silently dropped the null type. The field was not nullable in the generated spec.

The fix transforms the output to anyOf: [{ $ref }, { type: 'null' }], which is the correct OpenAPI 3.1 representation. The existing version processor handles 3.0 downgrade automatically (and is now also fixed to preserve outer schema metadata like description during that downgrade).

Closes #142

Type of Change

• 🐛 fix: Bug fix

Changes

• packages/openapi-core/src/schema/zod/zod-converter.ts: Two locations fixed:
  1. Direct SchemaIdentifier.nullable() / SchemaIdentifier.nullish() path (line ~1072): now produces anyOf: [{ $ref }, { type: 'null' }] instead of bare allOf: [{ $ref }]
  2. processZodChain nullable/nullish case (line ~1770): when schema.allOf is set, transforms to anyOf with null branch instead of silently skipping
• packages/openapi-core/src/openapi/version-processor.ts: downgradeSchemaForOpenApi30 now preserves outer metadata (e.g. description) when collapsing anyOf: [{ $ref }, { type: 'null' }] to { $ref, nullable: true }
• tests/integration/regressions/zod-nullability.test.ts: Two regression tests added for .nullish() and .nullable() on named schema references

Checklist

• pnpm check passes
• Tested locally (pnpm test:unit, pnpm test:integration and pnpm build)
• Documentation updated (if needed)