Fix GraspObjectNode initializaton order for ROS 2 humble

[hello-jkoubi]

Summary

Fix an initialization-order issue in grasp_object on ROS 2 Humble.

MoveBase was being constructed before the underlying ROS node was fully initialized. Since MoveBase declares ROS parameters during initialization, ros2 run stretch_demos grasp_object resulted in:

AttributeError: 'GraspObjectNode' object has no attribute '_parameter_overrides'

This change delays MoveBase initialization until after HelloNode.main() completes, ensuring the node is fully initialized before parameters are declared.

Testing Notes

grasp_object.py depends on stretch_funmap Python packages through the virtual environment workflow.

To test this PR, first apply the companion venv-wrapper PR and re-wrap the stretch_demos executables:

REx_amend_venv_execs.py stretch_demos --venv-package stretch_funmap --force

Then verify that the node starts successfully:

ros2 run stretch_demos grasp_object

Before this fix, the command failed during startup with:

AttributeError: 'GraspObjectNode' object has no attribute '_parameter_overrides'

After this change, grasp_object should initialize successfully.