[BUG] [PHP Symfony] Impossible to handle endpoints with "text/plain" or "image/png" response type

[gturri]

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

openapi-generator version

I tested with 7.12.0 and also with the latest master

OpenAPI declaration file content or url

This has 5 endpoints. Each of those endpoint illustrates the issue, but each with a different return type (so the fix can be tested against each)

openapi: 3.0.3
info:
  title: repro issue openapi-generator
  version: 0.0.1
paths:
  /get-bool:
    get:
      summary: get a boolean
      responses:
        "200":
          description: OK
          content:
            text/plain:
              schema:
                type: boolean
  /get-image:
    get:
      summary: get an image
      responses:
        "200":
          description: OK
          content:
            image/png:
              schema:
                type: string
                format: binary
  /get-string:
    get:
      summary: get a string
      responses:
        "200":
          description: OK
          content:
            text/plain:
              schema:
                type: string
  /get-int:
    get:
      summary: get an int
      responses:
        "200":
          description: OK
          content:
            text/plain:
              schema:
                type: integer
  /get-number:
    get:
      summary: get a number
      responses:
        "200":
          description: OK
          content:
            text/plain:
              schema:
                type: number

Generation Details

java -jar openapi-generator-cli.jar generate --git-user-id "some-user" --git-repo-id "some-project" -i "openapi.yaml" -g php-symfony -o "generated"

Steps to reproduce

Implement a simple symfony controller like this:

<?php
namespace App\Controller;

use OpenAPI\Server\Api\DefaultApiInterface;
use Endroid\QrCode\Builder\Builder;
use Endroid\QrCode\Writer\PngWriter;

class DefaultApi implements DefaultApiInterface {
    public function getBoolGet(int &$responseCode, array &$responseHeaders): array|object|null {
      return true;
    }

    // I'm using composer package "endroid/qr-code" to generate a png
    public function getImageGet(int &$responseCode, array &$responseHeaders): array|object|null {
		$builder = new Builder(writer: new PngWriter(), data: "some qr code");
		return $builder->build()->getString();
    }

    public function getIntGet(int &$responseCode, array &$responseHeaders): array|object|null {
        return 1312;
    }

    public function getNumberGet(int &$responseCode, array &$responseHeaders): array|object|null {
        return 13.12;
    }

    public function getStringGet(int &$responseCode, array &$responseHeaders): array|object|null {
        return "hello world!";
    }
}

then try to query any of those endpoints.

The generated code replies with a 406 instead of the expected response. (But there are actually 3 issues, and the 406 is just the first one that reveals itself. More details below).

Related issues/PRs

This is bit similar to #13334 but this report is more generic because #13334 identifies only one of the three actual issues (see below).
Similarly the open PR #15560 tries to address part of the issue, but it's only a partial fix of one of the issue

Suggest a fix

There are 3 things that needs to be fixed:

• The generated method Controller.getOutputFormat should be able to return more type than just application/json or application/xml (that's the reason why we have a 406)
• The generated methods of DefaultController should properly define the expected return type (see snippet above: what I return does not match array|object|null). This should be done by handling those other types in PhpSymfonyServerCodegen.java in the method postProcessOperationsWIthModels, when x-return-type is set
• in JmsSerializer.serialize it should not try to call ->serialize($data, ...) when the 2nd argument is null, otherwise it crashes at runtime (error 500) because the type of that 2nd argument is declared to be a string.

I'm going to submit shortly a PR that implements those 3 fixes