fix(player): 优化上下滑切集黑屏

test/player/player_logic_test.dart

@@ -594,8 +594,9 @@ void main() {
       await _drain(logic);
 
       expect(logic.session.currentIndex, 2);
-      // The last episode has no successor to warm.
-      expect(factory.created, hasLength(1));
+      // 最后一集没有下一集，但仍会预热上一集。
+      expect(factory.created, hasLength(2));
+      expect(logic.session.preparedPlayerAt(1), isNotNull);
 
       logic.dispose();
       await _drain(logic);