visual-only mesh fallback + visual/collision group split, refactors make_mjcf_from_robot_description.py to reuse the new logic

[saikishor]

Description

Second in the series splitting up #209 (see that PR for full context).

Builds on top of #264 by giving visual and collision geoms their own distinct roles instead of sharing one code path:

• Visual geometry is now purely for rendering (visual class). Visual meshes are kept as plain references — .stl/.obj untouched, only .dae converted (MuJoCo can't load DAE) — and are placed
  under assets/visual/. They are never decomposed, and the URDF material colour is carried over as the geom's rgba.
• Collision geometry drives physics (collision class). Collision primitives (box/sphere/cylinder/capsule) are used as-is; collision meshes are used as a single whole-mesh collision unless the link
  is explicitly listed in a decompose_mesh input, in which case they're convex-decomposed with obj2mjcf under assets/decomposed/. Decomposition is therefore opt-in per link rather than automatic.
• A mesh shared by a visual and a collision tag is converted once and reused for both.
• make_mjcf_from_robot_description.py was refactored to reuse this shared logic instead of duplicating it.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.62%. Comparing base (5e8920c) to head (113151f).

Additional details and impacted files

@@                              Coverage Diff                               @@
##           use/collision_tags/select_collision_meshes     #265      +/-   ##
==============================================================================
+ Coverage                                       81.39%   81.62%   +0.22%     
==============================================================================
  Files                                              30       30              
  Lines                                            5203     5267      +64     
  Branches                                          559      558       -1     
==============================================================================
+ Hits                                             4235     4299      +64     
  Misses                                            674      674              
  Partials                                          294      294              

Flag	Coverage Δ
unittests	81.62% <100.00%> (+0.22%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
...co_ros2_control/tests/test_urdf_to_mujoco_utils.py	99.83% <100.00%> (+<0.01%)	⬆️

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[scastro-nasa]

ok it is this branch that then causes some of our visual meshes to disappear... digging into it now.

EDIT: Also, the decomposed meshes do not show up at all in this branch?

[scastro-nasa]

@saikishor I put my changes up in #298

Just to check though, you do have upstack changes that prevent the visual and collision meshes from both being processed redundantly, right? That's #268 ?

[saikishor]

@saikishor I put my changes up in #298

Just to check though, you do have upstack changes that prevent the visual and collision meshes from both being processed redundantly, right? That's #268 ?

Yess exactly