feat(会话)：分页查询任务的历史残片/报告接口

StudySessionsService 新增：
- getTaskFragments(taskNum, page, size, keyword) → Page<FragmentsEntity>
- getTaskReports(taskNum, page, size, keyword) → Page<ReportsEntity>

通过子查询 session_num in (select session_num from study_sessions
where task_num = ?) 实现跨 session 聚合，keyword 支持 content
字段 LIKE 搜索。接口使用 MyBatis-Plus Page 分页。

控制器新增两个 GET 端点：
- /study-sessions/tasks/{taskNum}/fragments
- /study-sessions/tasks/{taskNum}/reports

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

@@ -2,7 +2,11 @@ package com.guo.learningprogresstracker.service;
 
 import com.google.protobuf.ServiceException;
 import com.guo.learningprogresstracker.dto.response.StudySessionResponse;
+import com.guo.learningprogresstracker.entity.StudyReportFragmentsEntity;
+import com.guo.learningprogresstracker.entity.StudyReportsEntity;
 import com.guo.learningprogresstracker.entity.StudySessionsEntity;
+import com.baomidou.mybatisplus.core.metadata.IPage;
+import com.baomidou.mybatisplus.extension.plugins.pagination.Page;
 import com.baomidou.mybatisplus.extension.service.IService;
 import com.guo.learningprogresstracker.exception.ErrorParameterException;
 
@@ -25,4 +29,8 @@ public interface StudySessionsService extends IService<StudySessionsEntity> {
     void pauseStudySession(String sessionNum, LocalDateTime endTime) throws ErrorParameterException, ServiceException;
 
     void continueStudySession(String sessionNum) throws ErrorParameterException, ServiceException;
+
+    Page<StudyReportFragmentsEntity> getTaskFragments(String taskNum, int page, int size, String keyword);
+
+    Page<StudyReportsEntity> getTaskReports(String taskNum, int page, int size, String keyword);
 }