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/controller/StudySessionController.java

@@ -4,8 +4,11 @@ import com.google.protobuf.ServiceException;
 import com.guo.learningprogresstracker.dto.request.EndedStudySessionRequest;
 import com.guo.learningprogresstracker.dto.request.UpsertExpectationRequest;
 import com.guo.learningprogresstracker.dto.response.StudySessionResponse;
+import com.baomidou.mybatisplus.extension.plugins.pagination.Page;
 import com.guo.learningprogresstracker.entity.CommonResult;
 import com.guo.learningprogresstracker.entity.StudyExpectationsEntity;
+import com.guo.learningprogresstracker.entity.StudyReportFragmentsEntity;
+import com.guo.learningprogresstracker.entity.StudyReportsEntity;
 import com.guo.learningprogresstracker.exception.ErrorParameterException;
 import com.guo.learningprogresstracker.service.StudyExpectationsService;
 import com.guo.learningprogresstracker.service.impl.StudySessionsServiceImpl;
@@ -107,4 +110,24 @@ public class StudySessionController {
     public CommonResult<String> getReportDraft(@PathVariable("sessionNum") String sessionNum) throws ErrorParameterException {
         return CommonResult.success("请求成功", studySessionsServiceImpl.generateReportDraft(sessionNum));
     }
+
+    /** 分页查询任务的残片历史 */
+    @GetMapping("/tasks/{taskNum}/fragments")
+    public CommonResult<Page<StudyReportFragmentsEntity>> getTaskFragments(
+            @PathVariable String taskNum,
+            @RequestParam(defaultValue = "1") int page,
+            @RequestParam(defaultValue = "10") int size,
+            @RequestParam(required = false) String keyword) {
+        return CommonResult.success(studySessionsServiceImpl.getTaskFragments(taskNum, page, size, keyword));
+    }
+
+    /** 分页查询任务的报告历史 */
+    @GetMapping("/tasks/{taskNum}/reports")
+    public CommonResult<Page<StudyReportsEntity>> getTaskReports(
+            @PathVariable String taskNum,
+            @RequestParam(defaultValue = "1") int page,
+            @RequestParam(defaultValue = "10") int size,
+            @RequestParam(required = false) String keyword) {
+        return CommonResult.success(studySessionsServiceImpl.getTaskReports(taskNum, page, size, keyword));
+    }
 }