Fix RViz Cartesian execute crash under sim-time mismatch

[chdkf]

Fix null pointer dereference in computeCartesianPlan() under sim_time

Problem

In simulation setups where /joint_states carries sim_time stamps and
RViz uses wall time, move_group_->getCurrentState() always times out and
returns nullptr (wall time ~1.78×10⁹ s vs sim time ~tens of seconds, so
the 1-second wait in waitForCurrentState(node->now(), 1s) never succeeds).
Dereferencing the returned pointer crashes the RViz process with SIGSEGV.

Fix

Replace the blocking call with the PlanningSceneMonitor's already-tracked
state, consistent with all five other current-state accesses in this file:

-rt.setRobotTrajectoryMsg(*move_group_->getCurrentState(), trajectory);
+rt.setRobotTrajectoryMsg(planning_display_->getPlanningSceneRO()->getCurrentState(), trajectory);

Why this is correct

• PlanningSceneMonitor maintains the current robot state continuously and does not block on timestamp matching.
• Every other call site in motion_planning_frame_planning.cpp already uses getPlanningSceneRO()->getCurrentState().
  This was the only exception.
• Behavior is identical in non-sim setups.

Reproduction

1. Launch MoveIt with Gazebo + ros2_control (use_sim_time=true), RViz with use_sim_time=false (default).
2. Compute a Cartesian path in the MotionPlanning panel.
3. Click Execute → RViz crashes with SIGSEGV before this patch.

[mergify]

Please target the main branch for development, we will backport the changes to humble for you if approved and if they don't break API.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 51.20%. Comparing base (e7004f4) to head (632d08b).

Additional details and impacted files

@@            Coverage Diff             @@
##           humble    #3722      +/-   ##
==========================================
- Coverage   51.21%   51.20%   -0.00%     
==========================================
  Files         407      407              
  Lines       30667    30667              
==========================================
- Hits        15704    15701       -3     
- Misses      14963    14966       +3     

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