[BUG][Typescript] Discriminator property name in oneOf is transformed incorrectly

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

The discriminator property name in oneOf is set to contentType if the content_type is used as a discriminator in the OpenAPI spec.
It would fail in runtime during response deserialization with the error TypeError: typeMap[type].getAttributeTypeMap is not a function.
Same might happen for serialization, didn't test that yet.

We need to make sure "content_type" is used, or that the "content_type" is transformed to "contentType" (might not be desirable).

openapi-generator version

7.11.0-SNAPSHOT
Exists at least since 7.10, possibly earlier.

OpenAPI declaration file content or url

openapi: 3.0.3
info:
  title: Discriminator Bug Example
  version: '1.0.0'
  description: A minimal example to demonstrate the bug with discriminatorName in oneOf template

paths:
  /test:
    get:
      summary: Test endpoint
      responses:
        '200':
          description: Success
          content:
            application/json:
              schema:
                $ref: '#/components/schemas/VendorInfo'

components:
  schemas:
    VendorInfo:
      title: Vendor Info
      description: |
        The information provided to the system from the vendor. Interpretation of this object is up to the client.
      type: object
      discriminator:
        propertyName: content_type
        mapping:
          application/json: '#/components/schemas/VendorJson'
          text/xml: '#/components/schemas/VendorXml'
      oneOf:
        - $ref: '#/components/schemas/VendorJson'
        - $ref: '#/components/schemas/VendorXml'

    VendorJson:
      type: object
      required:
        - content_type
        - json
        - vendor
      properties:
        content_type:
          type: string
          description: Describes the content-type encoding received from the vendor
          enum:
            - application/json
        json:
          type: object
          description: Data representation in JSON
        vendor:
          type: string
          example: EXAMPLE_VENDOR

    VendorXml:
      type: object
      required:
        - content_type
        - vendor
        - xml
      properties:
        content_type:
          type: string
          description: Describes the content-type encoding received from the vendor
          enum:
            - text/xml
        vendor:
          type: string
          example: EXAMPLE_VENDOR
        xml:
          type: string
          description: Data representation in XML

Generation Details

Generated with:
java -jar modules/openapi-generator-cli/target/openapi-generator-cli.jar generate -i test.yaml -g typescript -o test-gen/ --additional-properties=disallowAdditionalPropertiesIfNotPresent=false,legacyDiscriminatorBehavior=false,withNodeImports=true,platform=node

Steps to reproduce

• Generate
• Check the resulting generated models/VendorInfo.ts

It produces the following:

/**
* The information provided to the system from the vendor. Interpretation of this object is up to the client. 
*/
/**
 * @type VendorInfo
 * Type
 * @export
 */
export type VendorInfo = VendorJson | VendorXml;

/**
* @type VendorInfoClass
    * The information provided to the system from the vendor. Interpretation of this object is up to the client. 
* @export
*/
export class VendorInfoClass {
    static readonly discriminator: string | undefined = "contentType";

    static readonly mapping: {[index: string]: string} | undefined = {
        "application/json": "VendorJson",
        "text/xml": "VendorXml",
        "VendorJson": "VendorJson",
        "VendorXml": "VendorXml",
    };
}

It compiles fine, but will fail runtime since contentType wouldn't exist.

Related issues/PRs

https://github.com/OpenAPITools/openapi-generator/pull/20187

Suggest a fix

Reproduce the issue with the provided OpenAPI declaration and generation command, then inspect generated models/VendorInfo.ts. Trace how the OpenAPI discriminator propertyName content_type becomes the generated discriminator value contentType, using related PR #20187 as context. Done means generated TypeScript preserves the discriminator name and response deserialization no longer fails at runtime.