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

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

Start with the generated DefaultApi.java examples and compare ApiClient.parameterToPairs with parameterToMultiValueMap for the restclient, resttemplate, and webclient libraries. Reproduce the supplied OpenAPI document with the generation command, then inspect the relevant Java generator entry points and tests. Done means array values in an exploded parameter object produce repeated sort query parameters consistently with native and apache-httpclient.