[Php-Symfony] showcase recent features in petstore.yaml (#21286)

PR #21261 added support for endpoint with response of type text/plain
or even image/png.

This commit adds such endpoint so that:
- the way those are supported is clearer (as it is now directly visible
  in the generated sample files)
- if a future commit impacts this part of the generation it will be easier
  to assess that impact

samples/server/petstore/php-symfony/SymfonyBundle-php/Api/PetApiInterface.php

@@ -98,6 +98,23 @@ interface PetApiInterface
         array &$responseHeaders
     ): void;
 
+    /**
+     * Operation downloadFile
+     *
+     * downloads an image
+     *
+     * @param  int $petId  ID of pet to download an image from (required)
+     * @param  int     &$responseCode    The HTTP Response Code
+     * @param  array   $responseHeaders  Additional HTTP headers to return with the response ()
+     *
+     * @return mixed
+     */
+    public function downloadFile(
+        int $petId,
+        int &$responseCode,
+        array &$responseHeaders
+    ): mixed;
+
     /**
      * Operation findPetsByStatus
      *
@@ -150,6 +167,40 @@ interface PetApiInterface
         array &$responseHeaders
     ): array|object|null;
 
+    /**
+     * Operation petAge
+     *
+     * Get the age of the pet
+     *
+     * @param  int $petId  ID of pet (required)
+     * @param  int     &$responseCode    The HTTP Response Code
+     * @param  array   $responseHeaders  Additional HTTP headers to return with the response ()
+     *
+     * @return int
+     */
+    public function petAge(
+        int $petId,
+        int &$responseCode,
+        array &$responseHeaders
+    ): int;
+
+    /**
+     * Operation petAvailableForSale
+     *
+     * Whether the pet can currently be bought
+     *
+     * @param  int $petId  ID of pet (required)
+     * @param  int     &$responseCode    The HTTP Response Code
+     * @param  array   $responseHeaders  Additional HTTP headers to return with the response ()
+     *
+     * @return bool
+     */
+    public function petAvailableForSale(
+        int $petId,
+        int &$responseCode,
+        array &$responseHeaders
+    ): bool;
+
     /**
      * Operation updatePet
      *