fix(server): 修复 TestSubscribeTask_ConcurrentSafety 数据竞争问题

- 修复多个 goroutine 并发 append 同一 slice 导致的 DATA RACE
- 使用预分配 slice + 索引赋值替代并发 append
- 通过 go test -race 验证

server/internal/service/delivery_task_test.go

@@ -354,14 +354,14 @@ func TestSubscribeTask_ConcurrentSafety(t *testing.T) {
 	const goroutines = 50
 
 	// 并发订阅
-	cancels := make([]func(), 0, goroutines)
+	cancels := make([]func(), goroutines)
 	for i := 0; i < goroutines; i++ {
 		wg.Add(1)
-		go func() {
+		go func(idx int) {
 			defer wg.Done()
 			_, cancel := svc.SubscribeTask(taskID)
-			cancels = append(cancels, cancel)
-		}()
+			cancels[idx] = cancel
+		}(i)
 	}
 	wg.Wait()