[BUG][typescript-fetch] Default case for oneOf serialization method does not serialize correctly

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator?
• 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

In https://github.com/OpenAPITools/openapi-generator/pull/20983 was introduced that instead of throwing an error in the default case of a one of switch, the value itself is returned.
However this value is not serialized and instead directly returned.
This does lead to problem when for example a discriminator @type is used.
In typescript this is a field type which is serialized to @type during ToJSONTyped and thus is not serialized.

openapi-generator version

I am using 7.15, I don't know for which versions it worked

OpenAPI declaration file content or url

{
  "components": {
    "schemas": {
      "AbstractRequest": {
        "discriminator": {
          "propertyName": "@type"
        },
        "properties": {
          "@type": {
            "type": "string"
          }
        },
        "required": [
          "@type"
        ]
      },
      "SubRequest": {
        "allOf": [
          {
            "$ref": "#/components/schemas/AbstractRequest"
          },
          {
            "properties": {
              "@type": {
                "type": "string"
              },
              "id": {
                "type": "string"
              }
            },
            "type": "object"
          }
        ],
        "discriminator": {
          "propertyName": "@type"
        },
        "required": [
          "@type",
          "id"
        ]
      }
    }
  },
  "info": {
    "title": "API",
    "version": "0.1"
  },
  "openapi": "3.1.0"
}

The SubRequest.ts model contains the following method

export function SubRequestToJSONTyped(value?: SubRequest | null, ignoreDiscriminator: boolean = false): any {
    if (value == null) {
        return value;
    }

    if (!ignoreDiscriminator) {
        switch (value['type']) {
            default:
                return value;
        }
    }

    return {
        ...AbstractRequestToJSONTyped(value, true),
        '@type': value['type'],
        'id': value['id'],
    };
}

while the Request.ts contains

export function AbstractRequestToJSONTyped(value?: AbstractRequest | null, ignoreDiscriminator: boolean = false): any {
    if (value == null) {
        return value;
    }

    if (!ignoreDiscriminator) {
        switch (value['type']) {
            case 'SubRequest':
                return SubRequestToJSONTyped(value as SubRequest, ignoreDiscriminator);
            default:
                return value;
        }
    }

    return {
        
        '@type': value['type'],
    };
}

If we now serialize a SubRequest using SubRequestToJSONTyped we obtain

{
    "type": "SubRequest",
    "id": "id"
}

while we would expect

{
    "@type": "SubRequest",
    "id": "id"
}

Related issues/PRs

https://github.com/OpenAPITools/openapi-generator/pull/20983

Suggest a fix

I would expect that the default case should be

default:
    return SubRequestToJSONTyped(value, true);

Start by examining the generated SubRequest.ts and AbstractRequest.ts methods shown in the report, then review the related change in pull request 20983. Reproduce serialization with the supplied OpenAPI declaration and verify that the default discriminator path produces '@type' rather than 'type' while preserving 'id'.