docs: API 명세 실패 케이스 추가

Author: ysw789

src/main/java/dmu/dasom/api/domain/recruit/controller/RecruitController.java

@@ -2,7 +2,11 @@
 
 import dmu.dasom.api.domain.applicant.dto.ApplicantCreateRequestDto;
 import dmu.dasom.api.domain.applicant.service.ApplicantService;
+import dmu.dasom.api.domain.common.exception.ErrorResponse;
 import io.swagger.v3.oas.annotations.Operation;
+import io.swagger.v3.oas.annotations.media.Content;
+import io.swagger.v3.oas.annotations.media.ExampleObject;
+import io.swagger.v3.oas.annotations.media.Schema;
 import io.swagger.v3.oas.annotations.responses.ApiResponse;
 import io.swagger.v3.oas.annotations.responses.ApiResponses;
 import jakarta.validation.Valid;
@@ -23,8 +27,15 @@ public class RecruitController {
     // 지원하기
     @Operation(summary = "부원 지원하기")
     @ApiResponses(value = {
-            @ApiResponse(responseCode = "200", description = "지원 성공")
-    })
+            @ApiResponse(responseCode = "200", description = "지원 성공"),
+            @ApiResponse(responseCode = "400", description = "실패 케이스",
+                content = @Content(
+                        mediaType = "application/json",
+                        schema = @Schema(implementation = ErrorResponse.class),
+                        examples = {
+                            @ExampleObject(
+                                name = "학번 중복",
+                                value = "{ \"code\": \"C013\", \"message\": \"이미 등록된 학번입니다.\" }")}))})
     @PostMapping("/apply")
     public ResponseEntity<Void> apply(@Valid @RequestBody final ApplicantCreateRequestDto request) {
         applicantService.apply(request);