fix(ros2): correct PointField descriptors in DVS camera publisher

[youtalk]

Description

Fix a copy-paste bug in CarlaDVSCameraPublisher::SetPointCloudData where descriptor4 (for the pol field) was incorrectly assigned using descriptor3, causing:

1. descriptor3 (t field) to be overwritten with pol field values
2. descriptor4 to remain uninitialized

The PointCloud2 message published by the DVS camera sensor had incorrect field metadata, making it unusable by downstream ROS 2 consumers. I found it in the #9638 (comment).

Where has this been tested?

• Platform(s): Ubuntu 24.04
• Python version(s): N/A (C++ only)
• Unreal Engine version(s): 5.5

Possible Drawbacks

None. This is a straightforward bug fix with no behavioral change beyond correcting the PointField metadata.

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This change is 

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[Copilot]

Pull request overview

This PR fixes the ROS 2 DVS camera PointCloud2 field descriptors so the published message has correct metadata for downstream consumers.

Changes:

• Fixes a copy/paste error so the pol PointField descriptor is configured on descriptor4 (instead of overwriting descriptor3).
• Ensures the pol field descriptor is no longer left uninitialized.

Comments suppressed due to low confidence (1)

LibCarla/source/carla/ros2/publishers/CarlaDVSCameraPublisher.cpp:499

• PointCloud2 dimensional fields and steps appear internally inconsistent: the function copies height * width bytes into data, sets point_step to sizeof(DVSEvent), but then sets width directly on the message and computes row_step as width * point_size. With the current call site passing width = elements * sizeof(DVSEvent), this makes row_step equal to elements * sizeof(DVSEvent)^2, which won’t match the actual data buffer size. Please set pc.width to the number of points (likely elements with height = 1), compute row_step = pc.width * point_step, and size/copy the data buffer as elements * point_step bytes.

    descriptor4.count(1);

    const size_t point_size = sizeof(carla::sensor::data::DVSEvent);
    _point_cloud->_pc.width(width);
    _point_cloud->_pc.height(height);

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[JesusAnaya]

Reviewed. The descriptor block is correct: all four fields (x, y, t, pol) are now independently initialized and the offsets/datatypes match the DVSEvent memory layout. Clean fix.

[youtalk]

@Blyron @LuisPovedaCano This is a bug fix. Please review it.