feat: 新增碎片更新请求DTO和MapStruct映射

- 新建 UpdateFragmentsRequest，包含 @NotBlank 校验的 content 字段
- FragmentsConvert 新增 UpdateFragmentsRequest 到 Entity 的映射方法

src/main/java/com/guo/learningprogresstracker/dto/request/UpdateFragmentsRequest.java

@@ -0,0 +1,16 @@
+package com.guo.learningprogresstracker.dto.request;
+
+import jakarta.validation.constraints.NotBlank;
+import lombok.Data;
+
+/**
+ * @author guo
+ */
+@Data
+public class UpdateFragmentsRequest {
+    /**
+     * 残片内容,学习内容的描述
+     */
+    @NotBlank(message = "啊？无字天书？")
+    private String content;
+}

src/main/java/com/guo/learningprogresstracker/mapStruct/FragmentsConvert.java

@@ -1,6 +1,7 @@
 package com.guo.learningprogresstracker.mapStruct;
 
 import com.guo.learningprogresstracker.dto.request.CreateFragmentsRequest;
+import com.guo.learningprogresstracker.dto.request.UpdateFragmentsRequest;
 import com.guo.learningprogresstracker.entity.StudyReportFragmentsEntity;
 import org.mapstruct.Mapper;
 import org.mapstruct.ReportingPolicy;
@@ -11,4 +12,6 @@ public interface FragmentsConvert {
     FragmentsConvert MAPPER = Mappers.getMapper(FragmentsConvert.class);
 
     StudyReportFragmentsEntity toFragmentsEntity(CreateFragmentsRequest request);
+
+    StudyReportFragmentsEntity toFragmentsEntity(UpdateFragmentsRequest request);
 }