JTC test_success_multi_point_with_velocity_sendgoal is flaky

Describe the bug

    [  FAILED  ] PositionTrajectoryControllersActions/TestTrajectoryActionsTestParameterized.test_success_multi_point_with_velocity_sendgoal/2, where GetParam() = ({ "position" }, { "position", "velocity", "acceleration" })
    
     1 FAILED TEST
    -- run_test.py: return code 1
    -- run_test.py: inject classname prefix into gtest result file '/__w/ros2_controllers/ros2_controllers/target_ws/build/joint_trajectory_controller/test_results/joint_trajectory_controller/test_trajectory_actions.gtest.xml'
    -- run_test.py: verify result file '/__w/ros2_controllers/ros2_controllers/target_ws/build/joint_trajectory_controller/test_results/joint_trajectory_controller/test_trajectory_actions.gtest.xml'
  >>>
build/joint_trajectory_controller/test_results/joint_trajectory_controller/test_trajectory_actions.gtest.xml: 144 tests, 0 errors, 1 failure, 0 skipped
- joint_trajectory_controller.PositionTrajectoryControllersActions/TestTrajectoryActionsTestParameterized test_success_multi_point_with_velocity_sendgoal/2 (/__w/ros2_controllers/ros2_controllers/target_ws/src/ros-controls/ros2_controllers/joint_trajectory_controller/test/test_trajectory_actions.cpp:374)
  <<< failure message
    /__w/ros2_controllers/ros2_controllers/target_ws/src/ros-controls/ros2_controllers/joint_trajectory_controller/test/test_trajectory_actions.cpp:397
    Expected equality of these values:
      static_cast<int32_t>(expected_index)
        Which is: 0
      feedback_msg->index
        Which is: 1
  >>>

To Reproduce
https://github.com/ros-controls/ros2_controllers/actions/runs/32065820155/job/95497555839?pr=2558

Environment (please complete the following information):

• OS: RHEL
• Version lyrical

Start with the failing parameterized test test_success_multi_point_with_velocity_sendgoal in test/test_trajectory_actions.cpp, especially the assertions around lines 374 and 397. Reproduce it using the linked GitHub Actions run or the joint_trajectory_controller test suite, then trace why the feedback index intermittently differs from expected_index. Done means the test is stable and the suite passes for the reported case.