FIX #21407 - use referenced string enum schema in requestBody if available

[WannabeSoftwareEngineer]

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Description

This PR fixes the issue raised in #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.