feat: 新增碎片更新和会话查询的Service实现

- StudyReportFragmentsService 接口新增 updateFragments 和 getFragmentsBySession 方法
- ServiceImpl 实现碎片更新（校验存在性后更新）和按会话编号查询碎片列表

src/main/java/com/guo/learningprogresstracker/service/StudyReportFragmentsService.java

@@ -1,10 +1,13 @@
 package com.guo.learningprogresstracker.service;
 
 import com.guo.learningprogresstracker.dto.request.CreateFragmentsRequest;
+import com.guo.learningprogresstracker.dto.request.UpdateFragmentsRequest;
 import com.guo.learningprogresstracker.entity.StudyReportFragmentsEntity;
 import com.baomidou.mybatisplus.extension.service.IService;
 import com.guo.learningprogresstracker.exception.NotFindEntitiesException;
 
+import java.util.List;
+
 /**
 * @author guo
 * @description 针对表【study_report_fragments(记录学习过程中的学习内容报告残片)】的数据库操作Service
@@ -13,4 +16,8 @@ import com.guo.learningprogresstracker.exception.NotFindEntitiesException;
 public interface StudyReportFragmentsService extends IService<StudyReportFragmentsEntity> {
 
     void createFragments(CreateFragmentsRequest request) throws NotFindEntitiesException;
+
+    void updateFragments(Integer id, UpdateFragmentsRequest request) throws NotFindEntitiesException;
+
+    List<StudyReportFragmentsEntity> getFragmentsBySession(String sessionNum);
 }