[BE] refactor: dto 에 Nullable, validation 어노테이션 적용 (#410)

* refactor: 스프링 validation 어노테이션 추가

* refactor: Nullable 어노테이션 추가

* refactor: NotNull 추가

* refactor: NotNull에 메세지 추가

backend/src/main/java/reviewme/review/service/dto/request/CreateReviewAnswerRequest.java

@@ -1,18 +1,23 @@
 package reviewme.review.service.dto.request;
 
 import io.swagger.v3.oas.annotations.media.Schema;
+import jakarta.annotation.Nullable;
+import jakarta.validation.constraints.NotNull;
 import java.util.List;
 
 @Schema(name = "리뷰 답변 내용")
 public record CreateReviewAnswerRequest(
 
         @Schema(description = "질문 ID")
-        long questionId,
+        @NotNull(message = "질문 ID를 입력해주세요.")
+        Long questionId,
 
         @Schema(description = "선택된 옵션 ID 목록", nullable = true)
+        @Nullable
         List<Long> selectedOptionIds,
 
         @Schema(description = "답변 내용", nullable = true)
+        @Nullable
         String text
 ) {
 }

backend/src/main/java/reviewme/review/service/dto/request/CreateReviewRequest.java

@@ -1,15 +1,19 @@
 package reviewme.review.service.dto.request;
 
 import io.swagger.v3.oas.annotations.media.Schema;
+import jakarta.validation.constraints.NotBlank;
+import jakarta.validation.constraints.NotEmpty;
 import java.util.List;
 
 @Schema(name = "리뷰 생성 요청")
 public record CreateReviewRequest(
 
         @Schema(description = "리뷰 요청 코드")
+        @NotBlank(message = "리뷰 요청 코드를 입력해주세요.")
         String reviewRequestCode,
 
         @Schema(description = "답변 목록")
+        @NotEmpty(message = "답변 내용을 입력해주세요.")
         List<CreateReviewAnswerRequest> answers
 ) {
 }