[BUG][Jersey2][csharp-netcore]: Open API generator doesn't support oneof properties

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

Open API generator's Jersey2, csharp-netcore and python support polymorphic(oneof) classes. But they don't support when a particular property is of oneof type(universeCategory).

Supported one:

  "Universe":{
    "type":"object",
    "properties":{
      "universeCategory":{
        "$ref":"#/components/schemas/UniverseCategory"
      }
    },
    "additionalProperties":false
  },
  "UniverseCategory":{
    "oneOf":[
      {
        "$ref":"#/components/schemas/HalfUniverse"
      },
      {
        "$ref":"#/components/schemas/FullUniverse"
      }
    ],
    "discriminator":{
      "propertyName":"source",
      "mapping":{
        "HalfUniverse":"#/components/schemas/HalfUniverse",
        "FullUniverse":"#/components/schemas/FullUniverse"
      }
    }
  }

Unsupported one:

  "Universe":{
    "type":"object",
    "properties":{
      "universeCategory":{
        "oneOf":[
          {
            "$ref":"#/components/schemas/FullUniverse"
          },
          {
            "$ref":"#/components/schemas/HalfUniverse"
          }
        ],
        "discriminator":{
          "propertyName":"source",
          "mapping":{
            "FullUniverse":"#/components/schemas/FullUniverse",
            "HalfUniverse":"#/components/schemas/HalfUniverse"
          }
        }
      }
    }
  }

Swachbuckle generates the Open API specification only in the second format(unsupported format as above) and it is not possible to generate SDK.

openapi-generator version

5.3.1

OpenAPI declaration file content or url

Generation Details

Steps to reproduce

Related issues/PRs

https://github.com/OpenAPITools/openapi-generator/issues/7988

Suggest a fix

Start by reproducing the two schema forms in the issue for the Jersey2, csharp-netcore, and python generators, then compare the generated output with the supported referenced oneOf schema. Review related issue 7988 and generation behavior; done means property-level oneOf schemas generate usable SDKs for the reported formats.