[sensor_msgs] Fix row_step when resizing organized PointCloud2

[1visio]

Fixes #320

PointCloud2Modifier::resize(width, height) reused the flat resize(size)
path, then updated width and height without recomputing row_step. This
left row_step equal to the total data size instead of one row's byte size.

Set row_step to width * point_step and add a regression test covering a
non-square organized cloud and its backing data size.

Validation:

• git diff --check
• ROS 2 tests not run locally because colcon and ros2 are unavailable.

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