🐛 fix: 회원가입 로직에 닉네임 관련 예외처리 추가

Author: jiwon1217

backend/src/main/java/io/f1/backend/domain/user/app/UserService.java

@@ -30,8 +30,11 @@ public class UserService {
     @Transactional
     public CurrentUserAndAdminResponse signup(HttpSession session, SignupRequest signupRequest) {
         AuthenticationUser authenticationUser = extractSessionUser(session);
+        String nickname = signupRequest.nickname();
 
-        User user = initNickname(authenticationUser.userId(), signupRequest.nickname());
+        checkNickname(nickname);
+
+        User user = initNickname(authenticationUser.userId(), nickname);
         updateSessionAfterSignup(session, user);
 
         SecurityUtils.setAuthentication(user);