[BUG][typescript-fetch] Incorrect handling of arbitrary query parameters via `style: form` + `explode: true`

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

When an API endpoint specifies a combination of style: form and explode: true, it should generate code that handles adding query parameters exactly as they are and not nest them under the additionalProperties field; i.e., the URL used should be

http://api.example.com/v1/users?hello=world&foo=bar&additionalProp3=string

but the generated code generates this URL,

http://api.example.com/v1/users?additionalProperties[hello]=world&additionalProperties[foo]=bar&additionalProperties[additionalProp3]=string

This is the generated code block that causes the problem,

async usersGetRaw(requestParameters: UsersGetRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<object>> {
    const queryParameters: any = {};

    if (requestParameters['additionalProperties'] != null) {
        // this line,
        queryParameters['additionalProperties'] = requestParameters['additionalProperties'];

        // should be this instead (potentially)
        queryParameters = requestParameters['additionalProperties'];
    }

    const headerParameters: runtime.HTTPHeaders = {};

    const response = await this.request({
        path: `/users`,
        method: 'GET',
        headers: headerParameters,
        query: queryParameters,
    }, initOverrides);

    return new runtime.JSONApiResponse<any>(response);
}

openapi-generator version

7.17.0

OpenAPI declaration file content or url

openapi: 3.1.0
info:
  title: Sample API
  description: Optional multiline or single-line description in [CommonMark](http://commonmark.org/help/) or HTML.
  version: 1.0.0

servers:
  - url: http://api.example.com/v1
    description: Optional server description, e.g. Main (production) server
  - url: http://staging-api.example.com
    description: Optional server description, e.g. Internal staging server for testing

paths:
  /users:
    get:
      summary: Returns a list of users.
      description: Optional extended description in CommonMark or HTML.
      parameters:
        - in: query
          name: additionalProperties
          description: Allow arbitrary key name and values to be included as query parameters in the share-url.
          style: form
          explode: true
          schema:
            type: object
            additionalProperties:
              type: string
      responses:
        "200": # status code
          description: A JSON array of user names
          content:
            application/json:
              schema:
                type: object

Generation Details

No configuration file needed to reproduce this problem

Steps to reproduce

docker run --rm \
  -v ".:/local" \
  openapitools/openapi-generator-cli:latest generate \
  -g "typescript-fetch" \
  -i "/local/openapi.yml" \
  -o /local/src

Related issues/PRs

Suggest a fix

Start with the OpenAPI YAML in the issue and run the provided Docker typescript-fetch generation command. Inspect the generated usersGetRaw method and its query construction; done means the arbitrary object properties produce flat query parameters such as hello=world and foo=bar, rather than additionalProperties[...] keys.