Replace shared_ptr unique checks (backport #3793)

[mergify]

This is part of an effort to contribute RoboStack downstream patches back upstream.

Origin: focused portions of patch/ros-rolling-moveit-core.patch, patch/ros-rolling-moveit-planners-ompl.patch, and patch/ros-rolling-moveit-ros-planning.patch, authored by Daisuke Nishimatsu.

std::shared_ptr::unique() was removed in C++20. This replaces the remaining source-level ownership checks with equivalent use_count() comparisons while preserving the existing logic.

This intentionally leaves moveit_ros/robot_interaction untouched because that file is already covered by #3792.

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This is an automatic backport of pull request #3793 done by [Mergify](https://mergify.com).

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[130s]

Although I made this PR, for this one particularly I'm just not so sure if backporting (to jazzy or any older distros) makes sense.

[codecov]

Codecov Report

❌ Patch coverage is 75.00000% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 46.24%. Comparing base (a73060f) to head (923daa2).

Files with missing lines	Patch %	Lines
moveit_core/robot_state/src/attached_body.cpp	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##            jazzy    #3890      +/-   ##
==========================================
- Coverage   46.28%   46.24%   -0.03%     
==========================================
  Files         718      718              
  Lines       59182    59187       +5     
  Branches     7600     7599       -1     
==========================================
- Hits        27387    27367      -20     
- Misses      31630    31656      +26     
+ Partials      165      164       -1     

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