add robot push by adding velocity

[chuongnlvnx]

Previously, the push logic directly overwrote the robot's base velocity (x/y), which could cause unnatural motion.
This change modifies the behavior to add the random velocity to the robot's current base velocity instead, simulating a more physically consistent external disturbance.

This helps improve realism in domain randomization and encourages better policy robustness in RL training.