[Platform][OpenAI] simplify authentication error test structure

Remove unnecessary JSON fields from mock response to focus on the
essential error message extraction logic.

Author: vitality

src/platform/tests/Bridge/OpenAi/Gpt/ResultConverterTest.php

@@ -163,7 +163,7 @@ public function testThrowsAuthenticationExceptionOnInvalidApiKey()
         $httpResponse->method('getStatusCode')->willReturn(401);
         $httpResponse->method('getContent')->willReturn(json_encode([
             'error' => [
-                'message' => 'Invalid API key provided: sk-invalid'
+                'message' => 'Invalid API key provided: sk-invalid',
             ],
         ]));