重构项目结构，移除Assignment相关功能，优化Submission模块

Entities/DTO/Exam/ExamDto.cs

@@ -0,0 +1,44 @@
+using Entities.Contracts;
+using System;
+using System.ComponentModel.DataAnnotations;
+
+namespace Entities.DTO
+{
+    public class ExamDto
+    {
+        public Guid Id { get; set; } = Guid.NewGuid();
+
+        [Required(ErrorMessage = "考试标题是必填项。")]
+        [StringLength(255, ErrorMessage = "考试标题不能超过 255 个字符。")]
+        public string Title { get; set; }
+
+        public string Description { get; set; }
+
+		public byte TotalQuestions { get; set; }
+
+		public float Score { get; set; }
+
+		public string Name { get; set; }
+
+
+
+
+
+		[Required(ErrorMessage = "截止日期是必填项。")]
+		public DateTime DueDate { get; set; }
+
+		public Guid SubjectId { get; set; }
+
+        public Guid ExamTypeId { get; set; }
+
+        public Guid CreatorId { get; set; }
+
+		//public Guid ExamStructId { get; set; } 
+
+
+		public ExamQuestionDto ExamStruct { get; set; } = new ExamQuestionDto();
+
+        public DateTime CreatedAt { get; set; }
+        public DateTime UpdatedAt { get; set; }
+    }
+}