fix: remaining webrtc stream and test artifact changes

lib/features/desktop/desktop_client.dart

@@ -38,6 +38,9 @@ Future<MediaStream?> desktopRemoteVideoStreamForTrack(
   if (event.track.kind != 'video') {
     return null;
   }
+  if (event.streams.isNotEmpty) {
+    return event.streams.first;
+  }
   final stream = await createFallbackStream('xworkmate-remote-desktop');
   await stream.addTrack(event.track);
   return stream;

test/runtime/gateway_acp_client_auth_test.dart

@@ -1005,6 +1005,17 @@ void main() {
                       'output': 'recovered after running snapshot',
                       'turnId': 'turn-recovered-running',
                     },
+                  if (completed)
+                    'artifacts': <String, dynamic>{
+                      'items': <Map<String, dynamic>>[
+                        <String, dynamic>{
+                          'relativePath': 'exports/snapshot.md',
+                          'downloadUrl': 'https://xworkmate-bridge.svc.plus/artifacts/openclaw/download?sessionKey=unit-fixture-task-b&runId=turn-recovered-running&relativePath=exports%2Fsnapshot.md',
+                          'contentType': 'text/markdown',
+                          'sizeBytes': 64,
+                        },
+                      ],
+                    },
                 },
               }),
             );