[BUG][cpp-httplib-server] Top-level enum schema via $ref generates empty class with no enum members

[hp-rs]

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?

Description

When a top-level schema is defined as type: string with enum values and referenced via $ref, the cpp-httplib-server generator emits a class with only toJson() / fromJson() accessors but no actual enum class members and no enum values. The class is entirely hollow and cannot be used.
Inline enum definitions (defined directly on a property rather than in components/schemas) work correctly, but this is not acceptable when the same enum type is reused across multiple locations in the spec — deduplication via $ref is the correct OpenAPI pattern.

openapi-generator version

7.22.0

OpenAPI declaration file content or url

openapi: "3.0.0"
info:
  title: Enum Bug Reproduction
  version: "1.0.0"
paths:
  /example:
    get:
      operationId: getExample
      parameters:
        - in: query
          name: status
          schema:
            $ref: '#/components/schemas/StatusEnum'
      responses:
        '200':
          description: OK
components:
  schemas:
    StatusEnum:
      type: string
      enum:
        - ACTIVE
        - INACTIVE
        - PENDING

Generation Details

Generated class has no member variables:

/**
* This file is auto generated by OpenAPI Generator (https://openapi-generator.tech).
* https://openapi-generator.tech
* Do not edit the class manually.
*/

#pragma once
// System headers
#include <nlohmann/json.hpp>



namespace models {


class StatusEnum
{
public:

    StatusEnum();
    virtual ~StatusEnum() = default;

    // Getters and setters


    // JSON serialization using NLOHMANN INTRUSIVE macro (must be inside class to access private members)
    NLOHMANN_DEFINE_TYPE_INTRUSIVE(StatusEnum,
        )

private:
};


} // namespace models

Expected output: should have created an enum class with ACTIVE, INACTIVE and PENDING.

Steps to reproduce

docker run --rm -v $PWD:/local openapitools/openapi-generator-cli:v7.22.0 generate -g cpp-httplib-server -i local/test.yml -o ./local/out

Related issues/PRs

#958 — Similar symptom on cpp-restsdk
#10301 — $ref enum causes compilation error on cpp-restsdk

Suggest a fix

It seems that the cpp-httplib codegen is missing handling for top-level enums, as other C++ generators don't have this problem.