Trajectory blending with new trajectory deferral

[vedh1234]

Description

Partial port of the ROS 1 "update existing trajectory" behavior to ROS 2 joint_trajectory_controller.

Related to #84

What this PR does:

• Adds a new parameter allow_trajectory_blending (default: true)
• When enabled, a trajectory with a future header.stamp is deferred: the currently active trajectory continues executing until that start time arrives, then the new trajectory is installed
• This eliminates the legacy behavior where a future-stamped trajectory would immediately kill the active trajectory and create a slow spline ramp to the first point
• Deferred trajectories are correctly dropped on goal cancellation, controller deactivation, and controller reset
• Action Server feedback/tolerance checking is suppressed during the deferral period to prevent spurious aborts
• Immediate trajectories (stamp = 0 or stamp <= now) behave exactly as before (no behavioral change)

What this PR does NOT target: ( Compared to ROS1 blending feature )

• Does not allow omitted joints to continue their old trajectory after the new one starts (ROS 1 full blend). Omitted joints freeze at their current position via fill_partial_goal(), same as legacy ROS 2 behavior
• Does not handle speed scaling during the deferral period. The trigger uses wall-clock time, so if speed_scaling != 1.0, there will be a positional mismatch at handoff
• Does not track multiple action goals simultaneously. The new goal immediately preempts the old one at the Action Server level; only the physical trajectory execution is deferred

Is this user-facing behavior change?

Yes. When allow_trajectory_blending is true (default), the active trajectory will continue running until the new trajectory's start time.

Did you use Generative AI?

Yes, Google Gemini and Claude were used to make improvements in manually written code, architectural analysis, edge-case review.

Additional Information

• The existing disabled test test_execute_partial_traj_in_future referenced in Port over "update existing trajectory" to joint_trajectory_controller #84 has been enabled and passes

TODOs

To send us a pull request, please:

• Fork the repository.
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[codecov]

Codecov Report

❌ Patch coverage is 93.47826% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 85.58%. Comparing base (68630ae) to head (69e118d).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
...ory_controller/src/joint_trajectory_controller.cpp	94.44%	0 Missing and 2 partials ⚠️
...ntroller/test/test_trajectory_controller_utils.hpp	0.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2401      +/-   ##
==========================================
- Coverage   85.61%   85.58%   -0.04%     
==========================================
  Files         148      148              
  Lines       15837    15797      -40     
  Branches     1339     1339              
==========================================
- Hits        13559    13520      -39     
+ Misses       1792     1790       -2     
- Partials      486      487       +1     

Flag	Coverage Δ
unittests	85.58% <93.47%> (-0.04%)	⬇️

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% <100.00%> (+22.50%)	⬆️
...ory_controller/test/test_trajectory_controller.cpp	99.35% <100.00%> (+0.01%)	⬆️
...ntroller/test/test_trajectory_controller_utils.hpp	84.45% <0.00%> (ø)
...ory_controller/src/joint_trajectory_controller.cpp	84.31% <94.44%> (+0.32%)	⬆️

... and 4 files with indirect coverage changes

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[christophfroehlich]

Partial port of the ROS 1 "update existing trajectory" behavior to ROS 2 joint_trajectory_controller.

Fixes #84

For documentation here, what is different from the ROS 1 behavior/what is maybe missing to port?

Please also update the release_notes and update the documentation.