[BUG] [swift] URLSession library does not percent encode HTTP body for application/x-www-form-urlencoded

[Jonas1893]

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?
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Description

HTTP body for application/x-www-form-urlencoded requests does not get percent encoded using URLSession template. We detected this when migrating from Alamofire to URLSession template, in Alamofire this worked as described below

openapi-generator version

7.10.0

OpenAPI declaration file content or url

openapi: 3.0.3
info:
  title: Minimal spec
  description: |-
    Minimal spec for bug ticket
  version: 1.0.0
servers:
  - url: https://oauth2.basepath.com
paths:
  /any/oauth2.path:
    post:
      requestBody:
        content:
          application/x-www-form-urlencoded; charset=utf-8:
            schema:
              oneOf:
                - $ref: '#/components/schemas/LoginRequestModel'
      responses:
        '200':
          description: Successful operation

components:
  schemas:
    LoginRequestModel:
      type: object
      properties:
        client_id:
          type: string
        grant_type:
          type: string
          enum: ["refresh_token", "password", "urn:ietf:params:oauth:grant-type:device_code", "authorization_code"]
        password:
          type: string
        scope:
          type: string
        username:
          type: string
      required:
        [client_id, grant_type, password, scope, username]

Generation Details

config:

generatorName: swift6
library: urlsession
generateModelAdditionalProperties: false
additionalProperties:
  responseAs: AsyncAwait
  readonlyProperties: true
  nonPublicApi: false
  modelNamePrefix: API
  removeMigrationProjectNameClass: true

Since the code is the same I guess the same issue should happen with swift5

Steps to reproduce

Use the minimal spec with the provided config. The HTTP body of the request sent will be similar to:
client_id=7cfaf41e-aa18-40fd-a871-a21140d8e403&grant_type=password&password=foobarbaz&scope=openid email phone profile offline_access some-uid&username=+511287237674

Note that the spaces between the scopes and the + sign in the username are not percent encoded.

Expected output should be similar to:
client_id=7cfaf41e-aa18-40fd-a871-a21140d8e403&grant_type=password&password=foobarbaz&scope=phone%20some-uid%20openid%20profile%20email%20offline_access&username=%2B511287237674

Suggest a fix

The problem is that in private class FormURLEncoding -> func encode adds the non-escaped query to the httpBody:
urlRequest.httpBody = requestBodyComponents.query?.data(using: .utf8)
This means that any percent encoding needs to be done manually for the parameters. Changing this to
urlRequest.httpBody = requestBodyComponents.percentEncodedQuery?.data(using: .utf8) does not solve the issue entirely since the + sign will not be percent encoded.

I suggest to fix this by manually percent encoding each query item using .addingPercentEncoding(withAllowedCharacters: .alphanumerics) before adding them to requestBodyComponents.queryItems

If this is accepted as a bug I could contribute a fix for it.