[BUG][Go] Query parameter with oneOf enum doesn't work with defaultValue

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

When using an enum schema with oneOf and default, the new default value handling from #15126 causes the generated code to not build.

openapi-generator version

master @ 8b15d482

OpenAPI declaration file content or url

openapi: 3.1.0
info:
  description: 'Minimium viable reproduction for issue 16574'
  version: 1.0.0
  title: OpenAPI Petstore
paths:
  /pet/findByStatus:
    get:
      tags:
        - pet
      summary: Finds Pets by status
      description: Find pets by a single status
      operationId: findPetsByStatus
      parameters:
        - name: status
          in: query
          description: Status value that needs to be considered for filter
          schema:
            default: "available"
            oneOf:
              - $ref: '#/components/schemas/StatusType'
      responses:
        '200':
          description: successful operation
          content:
            application/xml:
              schema:
                type: array
                items:
                  $ref: '#/components/schemas/Pet'
            application/json:
              schema:
                type: array
                items:
                  $ref: '#/components/schemas/Pet'
        '400':
          description: Invalid status value
components:
  schemas:
    StatusType:
      type: string
      enum:
        - available
        - pending
        - sold
    Pet:
      title: a Pet
      description: A pet for sale in the pet store
      type: object
      required:
        - name
        - photoUrls
      properties:
        id:
          type: integer
          format: int64
        name:
          type: string
          example: doggie
        status:
          $ref: '#/components/schemas/StatusType'
      xml:
        name: Pet

Generation Details

> java -jar .\openapi-generator-cli.jar generate -g go -o petstore -i petstore.yaml  --package-name petstore -t C:\repos\openapi-generator\modules\openapi-generator\src\main\resources\go

Generates the following invalid go:

	if r.status != nil {
		parameterAddToHeaderOrQuery(localVarQueryParams, "status", r.status, "")
	} else {
		var defaultValue interface{} = available
		r.status = &defaultValue
	}

> go build
.\api_pet.go:80:34: undefined: available

Steps to reproduce

A query parameter using a schema with default and oneOf:

      parameters:
        - name: status
          in: query
          description: Status value that needs to be considered for filter
          schema:
            default: "available"
            oneOf:
              - $ref: '#/components/schemas/StatusType'

Related issues/PRs

Issue introduced by #15126

Suggest a fix

I tried running the generation with defaultValue=false but that didn't stop the default value code from being generated:

java -jar .\openapi-generator-cli.jar generate -g go -o petstore-i petstore.yaml  --additional-properties="defaultValue=false" --package-name petstore -t C:\repos\openapi-generator\modules\openapi-generator\src\main\resources\go

Inspect the Go generator templates under modules/openapi-generator/src/main/resources/go and trace default handling for the oneOf query parameter. Reproduce the issue with the provided OpenAPI YAML and generation command, then run go build on generated api_pet.go. Done means the generated default is a valid Go value and the client builds successfully.