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.

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[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?

[christophfroehlich]

@destogl I don't think it is a good idea to use the r6bot from the demos here unless we copy it. Moving the full description package incl. meshes will bloat this repo, and we can't make the demos a dependency of the test assets package. We can merge this as it is, because the kuka description was already in admittance_controller. If we need to, we can always copy the r6bot here and replace it.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.09%. Comparing base (7c5e767) to head (bc48177).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #3032   +/-   ##
=======================================
  Coverage   89.08%   89.09%           
=======================================
  Files         160      160           
  Lines       19751    19750    -1     
  Branches     1597     1597           
=======================================
  Hits        17596    17596           
- Misses       1488     1490    +2     
+ Partials      667      664    -3     

Flag	Coverage Δ
unittests	89.09% <ø> (+<0.01%)	⬆️

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