[bullet-featherstone] Fix attaching fixed joint

[iche033]

🦟 Bug fix

Fixes attaching fixed joint between 2 models.

Summary

When 2 models are attached by a fixed joint in bullet-featherstone, their rotations are not maintained (only the translation was set). This PR fixes the issue.

Test it

Before the fix, when you try to create a detachable joint in gz sim (which uses a fixed joint) between two models with rotations, the parent's rotation will be rotated to match the child's, making the models unstable.

I updated the joint_features test to use models with rotations.

To test with gz sim: run the box_attach.sdf world with bullet-featherstone:

gz sim -v 4 box_attach.sdf --physics-engine gz-physics-bullet-featherstone-plugin

Hit play to create the detachable joint and the boxes become unstable:

After the fix, the box should remain stable when the joint is created.

Checklist

• Signed all commits for DCO
• Added tests
• Updated documentation (as needed)
• Updated migration guide (as needed)
• Consider updating Python bindings (if the library has them)
• codecheck passed (See contributing)
• All tests passed (See test coverage)
• While waiting for a review on your PR, please help review another open pull request to support the maintainers

Note to maintainers: Remember to use Squash-Merge and edit the commit message to match the pull request summary while retaining Signed-off-by messages.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.33%. Comparing base (7d30e47) to head (8476306).

Additional details and impacted files

@@             Coverage Diff              @@
##           gz-physics7     #610   +/-   ##
============================================
  Coverage        78.32%   78.33%           
============================================
  Files              140      140           
  Lines             8069     8072    +3     
============================================
+ Hits              6320     6323    +3     
  Misses            1749     1749           

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