[BUG][typescript-fetch] oneOf ToJSON applies the date variant's toISOString() to every variant

Description

typescript-fetch, generator 7.14.0. A oneOf model where one variant has format: date-time properties and the others don't generates a ...ToJSONTyped that calls .toISOString() on the shared property names unconditionally. Serializing the number or string variant crashes with .toISOString is not a function. The generated discriminator type also collapses to the date variant's literal instead of the union.

Repro

components:
  schemas:
    Value:
      oneOf:
        - $ref: '#/components/schemas/NumberValue'
        - $ref: '#/components/schemas/DateValue'
    NumberValue:
      type: object
      required: [kind, value]
      properties:
        kind: { type: string, enum: [Number] }
        value: { type: number }
    DateValue:
      type: object
      required: [kind, value]
      properties:
        kind: { type: string, enum: [Date] }
        value: { type: string, format: date-time }

ValueToJSON({ kind: 'Number', value: 4 }) throws. Expected: dispatch per variant, or at least guard the Date conversion.

Generation Details

typescript-fetch, openapi-generator 7.14.0, spec produced by @typespec/openapi3.

Start by generating the typescript-fetch client from the YAML reproduction and inspect the generated ValueToJSON and ValueToJSONTyped functions. Trace how the oneOf variants and shared value property are serialized; done means NumberValue no longer calls toISOString(), DateValue still serializes its date, and the discriminator type preserves the union.