[BUG][Java][restclient,resttemplate,webclient] explode not applied to array values inside a parameter object

[wacker]

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?

Description

explode: true does not cause array values to be exploded when given inside a parameter object, as it does using the native or apache-httpclient library.

Example: PagingParams = { sort: [ 'amount,desc', 'name,asc'] } -- a typical Pageable object in Spring Web

Generated query string using library restclient ,resttemplate or webclient:
❌ /api/search?sort=amount,desc,name,asc

Generated query string using library native or apache-httpclient:
✅ /api/search?sort=amount,desc&sort=name,asc

openapi-generator version

v7.9.0

OpenAPI declaration file content or url

openapi: 3.0.3
info: { title: API, version: 1.0.0 }
paths:
  /api/search:
    get:
      parameters:
        - in: query
          name: PagingParams
          schema: { $ref: '#/components/schemas/PagingParams' }
          style: form
          explode: true
      responses:
        200: { description: OK }
components:
  schemas:
    PagingParams:
      type: object
      properties:
        page: { type: integer, format: int32, minimum: 0 }
        size: { type: integer, format: int32, minimum: 1 }
        sort: { type: array, items: { type: string } }

Generation Details

✅ generated DefaultApi.java using library native or apache-httpclient:

localVarQueryParams.addAll(ApiClient.parameterToPairs("multi", "sort", pagingParams.getSort()));

❌ generated DefaultApi.java using library restclient or webclient

queryParams.putAll(apiClient.parameterToMultiValueMap(null, "sort", pagingParams.getSort()));

❌ generated DefaultApi.java using using library resttemplate

localVarQueryParams.putAll(apiClient.parameterToMultiValueMap(null, "sort", pagingParams.getSort()));

Steps to reproduce

openapi-generator-cli generate -g java -i api-spec.yaml -o out-native --library native