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

Entities/DTO/ExamType/CreateExamTypeDto.cs

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+using System.ComponentModel.DataAnnotations;
+
+namespace Entities.DTO
+{
+    public class CreateExamTypeDto
+    {
+        [Required(ErrorMessage = "考试类型名称是必填项。")]
+        [StringLength(20, ErrorMessage = "考试类型名称不能超过 20 个字符。")]
+        public string ExamTypeName { get; set; }
+
+        public string Description { get; set; }
+    }
+}

Entities/DTO/ExamType/ExamTypeDto.cs

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+using System;
+using System.ComponentModel.DataAnnotations;
+
+namespace Entities.DTO
+{
+    public class ExamTypeDto
+    {
+        public Guid Id { get; set; }
+
+        [Required(ErrorMessage = "考试类型名称是必填项。")]
+        [StringLength(20, ErrorMessage = "考试类型名称不能超过 20 个字符。")]
+        public string Name { get; set; }
+
+        public string Description { get; set; }
+    }
+}

Entities/DTO/ExamType/ExamTypeResponseDto.cs

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+using System;
+using System.Collections.Generic;
+
+namespace Entities.DTO
+{
+    public class ExamTypeResponseDto
+    {
+        public Guid ExamId { get; set; }
+        public string ExamTypeName { get; set; }
+        public string Description { get; set; }
+
+        // Navigation properties for response
+        public int ExamCount { get; set; }
+        public List<string> ExamTitles { get; set; } = new List<string>();
+    }
+}

Entities/DTO/ExamType/UpdateExamTypeDto.cs

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+using System;
+using System.ComponentModel.DataAnnotations;
+
+namespace Entities.DTO
+{
+    public class UpdateExamTypeDto
+    {
+        [Required(ErrorMessage = "考试类型ID是必填项。")]
+        public Guid ExamTypeId { get; set; }
+
+        [Required(ErrorMessage = "考试类型名称是必填项。")]
+        [StringLength(20, ErrorMessage = "考试类型名称不能超过 20 个字符。")]
+        public string ExamTypeName { get; set; }
+
+        public string Description { get; set; }
+    }
+}