[BUG] Ruby Generator creates invalid constant from enum

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?
• [Optional] Sponsorship to speed up the bug fix or feature request (example)

Description

Generating a ruby client gem for nutanix APIs generates invalid ruby constants for enums that start with $

NameError:
  undefined local variable or method `_UNKNOWN' for NutanixClustermgmt::ClustermgmtV40ConfigHypervisorType:Class

        @all_vars ||= [AHV, ESX, HYPERV, XEN, NATIVEHOST, _UNKNOWN, _REDACTED].freeze
                                                          ^^^^^^^^

openapi-generator version

openapi-generator-cli-7.17.0.jar
Also tested with openapi-generator-cli-7.18.0-20251123.092543-63.jar

OpenAPI declaration file content or url

https://developers.nutanix.com/api/v1/namespaces/clustermgmt/versions/v4.0/yaml

    clustermgmt.v4.0.config.ErasureCodeStatus:
      type: string
      description: Indicates the current status value of Erasure Coding for the Storage
        Container.
      enum: 
      - NONE
      - "OFF" 
      - "ON"
      - $UNKNOWN
      - $REDACTED
      x-enumDescriptions:
        NONE: Option to ignore this option while creating the Storage Container.
        $UNKNOWN: |
          Unknown value.
        "OFF": Erasure coding is disabled for the Storage Container instance.
        $REDACTED: |
          Redacted value.
        "ON": Erasure coding is enabled for the Storage Container instance.

The spec validates with only warnings about unused models:

Validating spec (https://developers.nutanix.com/api/v1/namespaces/clustermgmt/versions/v4.0/yaml)
Warnings:
	- Unused model: clustermgmt.v4.0.config.UpdateRsyslogServerApiResponse
  ...
[info] Spec has 94 recommendation(s).

Generation Details

Steps to reproduce

java -jar openapi-generator-cli-7.17.0.jar generate --input-spec https://developers.nutanix.com/api/v1/namespaces/clustermgmt/versions/v4.0/yaml --generator-name ruby --config .openapi-config.json

$ cat .openapi-config.json 
{
  "gemName": "nutanix_clustermgmt",
  "gemAuthor": "ManageIQ Developers",
  "gemSummary": "Nutanix VMM API",
  "gemDescription": "Ruby gem for Nutanix Cluster Management APIs",
  "gemLicense": "Apache-2.0",
  "useAutoload": true
}

$ bin/console
irb(main):001> NutanixClustermgmt::ClustermgmtV40ConfigHypervisorType.all_vars
/home/grare/adam/src/manageiq/nutanix_clustermgmt-sdk-ruby/lib/nutanix_clustermgmt/models/clustermgmt_v40_config_hypervisor_type.rb:27:in `all_vars': undefined local variable or method `_UNKNOWN' for class NutanixClustermgmt::ClustermgmtV40ConfigHypervisorType (NameError)

      @all_vars ||= [AHV, ESX, HYPERV, XEN, NATIVEHOST, _UNKNOWN, _REDACTED].freeze
                                                        ^^^^^^^^
	from (irb):1:in `<main>'
	from <internal:kernel>:187:in `loop'
	from bin/console:15:in `<main>'

Related issues/PRs

Suggest a fix

Reproduce the issue with the documented openapi-generator CLI command, the Nutanix OpenAPI URL, and generator-name ruby, then inspect the generated clustermgmt_v40_config_hypervisor_type.rb model. Trace how enum values beginning with "$" become constants and add coverage for the affected enum; done means the generated Ruby model loads and all_vars works without NameError.