feat(cameras): prefer MJPG when fourcc unspecified in ZMQ (#4277)

* feat(cameras): prefer MJPG when fourcc unspecified; allow fourcc in ZMQ image server

With no fourcc set, OpenCV's V4L2 auto-negotiation selects uncompressed
YUYV when the camera offers it: ~16x the USB bandwidth of MJPG for
identical frames, which silently caps the frame rate on shared USB 2.0
buses (e.g. a Raspberry Pi with multiple cameras).

OpenCVCamera now prefers MJPG when config.fourcc is None: it requests
MJPG and keeps it only if the camera reports support (read-back),
falling back to the camera's own negotiation otherwise. An explicit
config.fourcc always takes precedence, so opting out is fourcc="YUYV".
The preference mirrors the existing pre/post size-and-fps ordering to
preserve the Windows DSHOW FOURCC-override handling.

Also adds a fourcc passthrough to the ZMQ image server, which previously
could not request a pixel format at all.

Behavior change: cameras that relied on implicit uncompressed capture
now receive camera-side JPEG unless fourcc is set explicitly.

* chore(cameras): address feedback

---------

Co-authored-by: Xingdong Zuo <18168681+zuoxingdong@users.noreply.github.com>

src/lerobot/cameras/zmq/image_server.py

@@ -102,6 +102,7 @@ class ImageServer:
                 fps=self.fps,
                 width=shape[1],
                 height=shape[0],
+                fourcc=cfg.get("fourcc", "MJPG"),
                 color_mode=ColorMode.RGB,
             )
             camera = OpenCVCamera(cam_config)