refactor : model 변경

Author: samuel426

backend/src/main/java/com/backend/domain/evaluation/service/impl/OpenAiGateway.java

@@ -43,7 +43,7 @@ public String complete(String content, String prompt) {
         );
 
         var params = ResponseCreateParams.builder()
-                .model(ChatModel.GPT_4_1)
+                .model(ChatModel.GPT_5_NANO)
                 .input(input)
                 .build();