Use referenced string enum schema in requestBody if available

[WannabeSoftwareEngineer]

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Description

This PR fixes #21407.
The core of this change is to treat a referenced string enum schema as a model rather than a primitive.

To achieve this, the updateRequestBodyForString method has been updated to accept the schema's component name. When this name is present, we delegate processing to addBodyModelSchema. This mirrors the existing logic that is already present in updateRequestBodyForObject, resulting in the proper enum type in the generated API.

The behavior for primitive or inline strings remains unchanged.

Breaking change

Since this modifies the generated method signatures, the new behavior is introduced behind a feature flag named useStringEnumSchemaRefForRequestBody to ensure back compatibility with existing clients.

[WannabeSoftwareEngineer]

Tagging some members of the Java committee
@martin-mfg @lwlee2608

I would appreciate a few moments of your time to share any feedback on the proposed changes. Thank you.

[Mattias-Sehlstedt]

Hello, could you please adjust the message so that it aligns with this? It would be helpful since it would make the issues page show that there already exists a suggested solution for the raised issue.

(the icon marked in red is what is added)

[WannabeSoftwareEngineer]

This PR should have already been linked to the corresponding issue.

Let me know if you meant something else.

[wing328]

thanks for the pr

what about considering this as a bug fix without the option to start with as we want to avoid option overload (too many options)?

if later users need ways to fallback, of course one way is to update the spec to remove enum from the string body definition.

[WannabeSoftwareEngineer]

I also prefer to keep the number of options minimal, but I initially introduced the flag to avoid a breaking change for existing users.

Since you're okay with treating this as a bug fix and going ahead without the flag, I'll update the MR to remove the option accordingly.