[BUG] [typescript-fetch] Union types mixing objects and primitives generate invalid types

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• Have you tested with the latest master to confirm the issue still exists?
• Have you searched for related issues/PRs?
• What's the actual output vs expected output?
• [Optional] Sponsorship to speed up the bug fix or feature request (example)

Description

When generating a client with typescript-fetch, union types between primitives and objects are not generated correctly.

openapi-generator version

Master: e3121af4b41f6ae37a2f5721c004ab982af8a1c3

OpenAPI declaration file content or url

openapi: 3.0.0
info:
  title: MixedUnionTypes
  version: 0.0.1

paths: {}
components:

  schemas:

    MixedUnionTypes:
      type: object
      properties:
        fields:
          type: array
          items:
            oneOf:
              - $ref: "#/components/schemas/SimpleFieldParameter"
              - $ref: "#/components/schemas/AdvancedFieldParameter"
    SimpleFieldParameter:
      type: string
    AdvancedFieldParameter:
      type: object
      properties:
        name:
          type: string
        anotherProp:
          type: string
      required:
        - name

Generation Details

Steps to reproduce

Related issues/PRs

Suggest a fix

I'll create a PR to handle primitive first and then fallback to JSON conversion

Start by reproducing the issue with the supplied MixedUnionTypes OpenAPI YAML and the typescript-fetch generator. Inspect how the generator handles the oneOf union between SimpleFieldParameter and AdvancedFieldParameter; done means the generated TypeScript represents both the primitive and object alternatives without invalid types, with a regression test for this schema.