fix(lekiwi): pin MJPG capture in the default camera config (#4083)

Without an explicit fourcc, OpenCV's V4L2 auto-negotiation selects
uncompressed YUYV when the camera offers it: ~147 Mbps per camera at
640x480@30 on the Pi's shared USB 2.0 bus, versus ~9 Mbps as MJPG for
identical frames. Two default cameras already exceed the bus, so frame
rates sag silently.

Pinning fourcc="MJPG" is backward compatible: it is an existing
OpenCVCameraConfig field, frames still arrive as BGR ndarrays, user
configs that set their own fourcc are unaffected, and a camera without
MJPG fails loudly at validation rather than degrading silently.

Co-authored-by: Xingdong Zuo <18168681+zuoxingdong@users.noreply.github.com>
Co-authored-by: Steven Palma <imstevenpmwork@ieee.org>

src/lerobot/robots/lekiwi/config_lekiwi.py

@@ -23,10 +23,20 @@ from ..config import RobotConfig
 def lekiwi_cameras_config() -> dict[str, CameraConfig]:
     return {
         "front": OpenCVCameraConfig(
-            index_or_path="/dev/video0", fps=30, width=640, height=480, rotation=Cv2Rotation.ROTATE_180
+            index_or_path="/dev/video0",
+            fps=30,
+            width=640,
+            height=480,
+            fourcc="MJPG",
+            rotation=Cv2Rotation.ROTATE_180,
         ),
         "wrist": OpenCVCameraConfig(
-            index_or_path="/dev/video2", fps=30, width=480, height=640, rotation=Cv2Rotation.ROTATE_90
+            index_or_path="/dev/video2",
+            fps=30,
+            width=480,
+            height=640,
+            fourcc="MJPG",
+            rotation=Cv2Rotation.ROTATE_90,
         ),
     }