Mutation of the received response (Without violating PSR)

Question

I am making an SDK client application that will work with a third-party API, and during the development process I try to adhere to PSR standards. I have two questions that I don't fully understand, and I hope to get an answer.

Composer depends:

{
    "require": {
        "php": ">=8.2",
        "php-http/client-common": "^2.5",
        "php-http/discovery": "^1.14.3",
        "php-http/httplug": "^2.3",
        "php-http/multipart-stream-builder": "^1.2",
        "psr/http-client-implementation": "^1.0",
        "psr/http-factory-implementation": "^1.0",
        "psr/http-message": "^1.0.1"
    },
    "require-dev": {
        "phpunit/phpunit": "^8.5",
        "guzzlehttp/guzzle": "^7.5",
        "guzzlehttp/psr7": "^2.4.1",
        "http-interop/http-factory-guzzle": "^1.2"
    }
}

Application tree:

./src/
├── Api
│   ├── AbstractApi.php
│   ├── Achievement.php
│   ├── AchievementInterface.php
│   ├── Clan.php
│   ├── ClanInterface.php
│   ├── Game.php
│   ├── GameInterface.php
│   ├── Rating.php
│   ├── RatingInterface.php
│   ├── User.php
│   ├── UserInterface.php
│   ├── Weapon.php
│   └── WeaponInterface.php
├── Client.php
├── ClientExtraInterface.php
├── Enum
│   ├── GameClassEnum.php
│   ├── GameClassList.php
│   ├── Http
│   │   └── RegionList.php
│   └── Leagues.php
├── Exception
│   ├── Response
│   │   ├── ApiException.php
│   │   └── InvalidJsonException.php
│   ├── UndefinedBranchException.php
│   └── UnknownMethodCallException.php
├── Helper
│   └── QueryParamsHelper.php
└── HttpClient
    ├── Builder.php
    ├── Message
    │   └── ResponseMediator.php
    └── Plugin
        ├── BypassTimeoutResponsePlugin.php
        └── ServerSupportPlugin.php

1. In which directory is it correct to store interfaces for classes from the src/Api directory. Is it correct to leave it as it is now in the tree, or should it be moved to a separate directory, for example, src/Contacts?

2. The question is about the mutation of the received answer. For example, how can I, adhering to the PSR, implement the modification of the received response from the API, and is it acceptable to change the received response at all. Can I do this within the ResponseMediator class?

   final class ResponseMediator
   {
       /**
        * @throws ApiException|InvalidJsonException
        */
       public static function getContent(ResponseInterface $response): array
       {
           $body = $response->getBody()->__toString();
   
           if (!str_starts_with($response->getHeaderLine('Content-Type'), 'application/json')) {
               throw new ApiException(sprintf('The response is not JSON: %s', $body));
           }
   
           try {
               /** @var array<string|int, string|int> $content */
               return json_decode($body, true, 512, JSON_THROW_ON_ERROR);
           } catch (JsonException $e) {
               throw new InvalidJsonException(
                   sprintf('Invalid JSON: %s', $e->getMessage())
               );
           }
       }
   }
   

In the last question, I am interested in the correctness of the approach to such manipulations. In fact, I can implement this using plugins (src/HttpClient/Plugin/. I hope you can help me figure it out, thank you.