fix: migrate active goal and tolerances to thread safe box in JTC

[SouriRishik]

Description

This PR migrates the JointTrajectoryController (JTC) to the modern RealtimeThreadSafeBox API to replace the deprecated RealtimeBuffer for the active goal handle and tolerances.

The migration from stale PR #1480 has been completed and brought up to date with the following fixes:

1. Fixed a bug in all five try_set() calls where the lambda parameter was passed by value ([](auto goal)). This caused the compiler to copy the smart pointer, resulting in the active goal handle never being cleared from the box and causing subsequent actions/tests to fail or timeout.
2. Resolved the merge conflict in preempt_active_goal() by merging the new box .set() API with the master branch's preemption abort-result delivery logic (active_goal->runNonRealtime()).
3. Added a clean target fallback check in CMakeLists.txt to define the ros2_control_test_assets::ros2_control_test_assets target if it is not exported, ensuring compatibility with older ROS environments (such as Humble).

This finishes the stale PR #1480 and resolves the test failures.

Is this user-facing behavior change?

No

Did you use Generative AI?

No

Additional Information

Verified locally that the pre-commit checks run and pass successfully:

pre-commit run --files joint_trajectory_controller/src/joint_trajectory_controller.cpp joint_trajectory_controller/include/joint_trajectory_controller/joint_trajectory_controller.hpp joint_trajectory_controller/test/test_trajectory_controller_utils.hpp

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.85%. Comparing base (c7b97ca) to head (96eaa97).
⚠️ Report is 4 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2484      +/-   ##
==========================================
+ Coverage   86.83%   86.85%   +0.02%     
==========================================
  Files         148      148              
  Lines       16273    16278       +5     
  Branches     1368     1369       +1     
==========================================
+ Hits        14131    14139       +8     
+ Misses       1635     1631       -4     
- Partials      507      508       +1     

Flag	Coverage Δ
unittests	86.85% <100.00%> (+0.02%)	⬆️

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

Files with missing lines	Coverage Δ
...jectory_controller/joint_trajectory_controller.hpp	62.50% <ø> (ø)
...ory_controller/src/joint_trajectory_controller.cpp	85.43% <100.00%> (+0.19%)	⬆️
...ntroller/test/test_trajectory_controller_utils.hpp	83.72% <100.00%> (ø)

... and 1 file with indirect coverage changes

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[SouriRishik]

Hi @christophfroehlich, could you please review?