feat(app): 完善播放协议搜索与空状态

test/player/player_video_surface_test.dart

@@ -69,6 +69,51 @@ void main() {
     expect(_opacity(tester), 0);
     expect(find.byKey(const Key('player-debug-video')), findsNothing);
   });
+
+  testWidgets('should_cover_new_video_only_while_handoff_is_held', (
+    tester,
+  ) async {
+    final current = VideoPlayerController.networkUrl(
+      Uri.parse('http://192.168.31.187:8081/files/720.m3u8'),
+    );
+    final previous = VideoPlayerController.networkUrl(
+      Uri.parse('http://192.168.31.187:8081/files/360.m3u8'),
+    );
+    addTearDown(current.dispose);
+    addTearDown(previous.dispose);
+    const ready = VideoPlayerValue(
+      duration: Duration(minutes: 1),
+      size: Size(1280, 720),
+      position: Duration(seconds: 3),
+      isInitialized: true,
+      isPlaying: true,
+    );
+    current.value = ready;
+    previous.value = ready;
+
+    Future<void> pump(VideoPlayerController? handoff) {
+      return tester.pumpWidget(
+        MaterialApp(
+          home: Scaffold(
+            body: PlayerVideoSurface(
+              controller: current,
+              handoff: handoff,
+              videoUrl: current.dataSource,
+              theater: false,
+            ),
+          ),
+        ),
+      );
+    }
+
+    await pump(previous);
+    expect(find.byKey(const Key('player-video-handoff')), findsOneWidget);
+    expect(_opacity(tester), 1);
+
+    await pump(null);
+    expect(find.byKey(const Key('player-video-handoff')), findsNothing);
+    expect(find.byType(VideoPlayer), findsOneWidget);
+  });
 }
 
 Future<VideoPlayerController> _pumpSurface(