feat(typescript-zod): preserve named object unions

[louis-bompart]

Description

Enables supportsUnionsWithMultipleObjectTypes for the TypeScript Zod target and extends the named-object-union regression test to cover Zod output.

Related Issue

Depends on #3061, which fixes #1646. This is intentionally a stacked draft PR.

Motivation and Context

#3061 adds guarded support for explicitly named, disjoint object alternatives. TypeScript Zod can represent those alternatives directly with z.union, so it can safely opt in without renderer changes.

Previous Behaviour / Output

Zod received the merged optional-properties object produced by the core union-flattening pass. Distinct alternatives such as Foo and Bar were not retained as separate schemas.

New Behaviour / Output

The generated Zod output preserves both schemas and references them from the containing schema:

export const FooSchema = z.object({ "foo": z.string() });
export const BarSchema = z.object({ "bar": z.string() });
export const ContainerSchema = z.object({
    "op": z.union([FooSchema, BarSchema]),
});

The Zod fixture also rejects an invalid union member.

How Has This Been Tested?

Tested locally with Node.js 24.18.0:

• npm run build
• npm run test:unit
• QUICKTEST=true FIXTURE=schema-typescript-zod npm run test:fixtures -- test/inputs/schema/named-class-union.schema

The focused fixture round-trips both valid alternatives and verifies that the invalid sample fails.

Screenshots (if appropriate):

Not applicable.