feat: 有效学习时间不足10分钟不计入总学习时间

- StudySessionsEntity: 新增 MIN_EFFECTIVE_TIME_SECONDS 常量，
  endedStudySession() 中检查 effectiveTime，不足则清零
- StudySessionsService: endedStudySession 返回类型从 void 改为 String

src/main/java/com/guo/learningprogresstracker/entity/StudySessionsEntity.java

@@ -132,6 +132,11 @@ public class StudySessionsEntity extends BaseEntity implements Serializable {
         }
     }
 
+    /**
+     * 有效学习时间的最小阈值（秒），低于此值不计入总学习时间
+     */
+    private static final double MIN_EFFECTIVE_TIME_SECONDS = 10 * 60;
+
     /**
      * 结束会话
      */
@@ -140,7 +145,12 @@ public class StudySessionsEntity extends BaseEntity implements Serializable {
             log.warn("不应出现的情况：结束了一个状态为【{}】的学习会话",this.getSessionState());
         }else {
             if (this.getSessionState().equals(StudySessionStateEnum.ONGOING.name())) {
-                this.pausedStudySession(endTime);
+                this.pausedStudySession(LocalDateTime.now());
+            }
+            if (this.getEffectiveTime() < MIN_EFFECTIVE_TIME_SECONDS) {
+                log.info("会话[{}]有效学习时间({}秒)不足10分钟，不计入总学习时间", this.getSessionNum(), this.getEffectiveTime());
+                this.setEffectiveTime(0);
+                this.setEffectivenessRatio(0);
             }
             this.setSessionState(StudySessionStateEnum.ENDED.name());
         }

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

@@ -20,7 +20,7 @@ public interface StudySessionsService extends IService<StudySessionsEntity> {
 
     ArrayList<String> getAllFragments(String sessionNum) throws ErrorParameterException;
 
-    void endedStudySession(String sessionNum, String content) throws ErrorParameterException;
+    String endedStudySession(String sessionNum, String content) throws ErrorParameterException;
 
     void pauseStudySession(String sessionNum, LocalDateTime endTime) throws ErrorParameterException, ServiceException;