Add 6D robot description to ros2_control_test_assets

[naitikpahwa18]

Description

Adds test_asset_6d_robot_description.hpp to ros2_control_test_assets package to make it reusable across multiple controllers.

Related Issues

Part of fix for ros-controls/ros2_controllers#450

Additional context

This PR should be merged in coordination with the ros-controls/ros2_controllers#2173

[Copilot]

Pull request overview

Adds a new reusable test asset header to ros2_control_test_assets that provides a 6-DOF robot URDF and matching SRDF as embedded string constants, intended for reuse across multiple controller test suites.

Changes:

• Added test_asset_6d_robot_description.hpp containing valid_6d_robot_urdf and valid_6d_robot_srdf string assets.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

[destogl]

I think we should switch to another robot description that we are using in the ros2_control_demos. Here it is: https://github.com/ros-controls/ros2_control_demos/tree/master/ros2_control_demo_description/r6bot

@christophfroehlich, @bmagyar and @saikishor should we copy this description or move it here and then reference it always from test assets? Or is maintaining two copies OK in this case?

[destogl]

Suggested change

	// Copyright (c) 2021, Stogl Robotics Consulting UG (haftungsbeschränkt)
	// Copyright (c) 2021, b»robotized group