[BUG][TypeScript-Fetch] The propertyName of the OneOf discriminator is incorrect.

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

openapi-generator version

6.6.0

OpenAPI declaration file content or url

components:
  schemas:
    UnionTest:
      oneOf:
        ...
      discriminator:
        propertyName: Type
        mapping":
          ...

Generation Details

legacyDiscriminatorBehavior: false

Output

function UnionTestFromJSONTyped(json, ignoreDiscriminator) {
    switch (json['type']) {
        ...

Expected Output

function UnionTestFromJSONTyped(json, ignoreDiscriminator) {
    switch (json['Type']) {
        ...

propertyName is converted to lowerSnakeCase.
Do not change the json property name

Temporary solution

modelPropertyNaming: original

Start with the TypeScript-Fetch generator and reproduce the issue using the supplied OpenAPI schema, propertyName setting, and legacyDiscriminatorBehavior configuration. Compare the generated discriminator access with the expected TypeScript output and verify that the JSON property name remains unchanged without relying on modelPropertyNaming: original.