Fix display.launch.py default model path handling

[shreyasathreyav]

While running the urdf_tutorial on a newer setup (ROS 2 Kilted on Ubuntu 24.04),
launching the tutorial failed unless the model path was manually prefixed with
urdf/.

The issue was caused by the default model launch argument being constructed
using a PathJoinSubstitution and then stringified. On newer ROS 2 / Python
versions, LaunchArgument defaults are treated strictly as strings, which led to
invalid URDF paths being passed to xacro.

This change:

• Uses a plain string as the default value for the model argument
• Prepends the urdf/ directory at the tutorial level when invoking urdf_launch

This keeps urdf_launch generic while fixing the tutorial’s default behavior and
avoids requiring users to manually adjust paths.

Tested on ROS 2 Kilted (Ubuntu 24.04).

[shreyasathreyav]

The Ubuntu Noble PR jobs appear to fail during repository checkout (git init fails to execute) before any build or test steps run. This looks like a Jenkins agent issue rather than a code issue. Happy to re-run CI if needed.