Fix robot_state_publisher model directory path and naming consistency for multi-robot support in vrx_gz/launch.py

[xiaolixi]

Description
branch: jazzy
When attempting to spawn multiple WAM-V models simultaneously in the VRX Gazebo simulation, the robot_state_publisher node fails to work correctly for the second robot. I've identified that the issue stems from hardcoded paths and naming conventions at line ~372 in vrx_gz/src/vrx_gz/launch.py.

Current Implementation (Line ~372)

# robot_state_publisher (tf for wamv)
model_dir = os.path.join(get_package_share_directory('vrx_gazebo'), 'models/wamv/tmp')
urdf_file = os.path.join(model_dir, 'model.urdf')
with open(urdf_file, 'r') as infp:
    robot_desc = infp.read()
params = {'use_sim_time': use_sim_time, 'frame_prefix': 'wamv/', 'robot_description': robot_desc}
nodes.append(Node(package='robot_state_publisher',
                      executable='robot_state_publisher',
                      output='both',
                      parameters=[params],
                      remappings=[('/joint_states', '/wamv/joint_states')]))

Problems Identified

1. Hardcoded model path: The path models/wamv/tmp is hardcoded, preventing the use of other robot models or custom model names
2. Inconsistent naming: frame_prefix and remapping topics are hardcoded to wamv/ instead of using the model.model_name variable

Proposed Fix

# robot_state_publisher (tf for wamv)
model_dir = os.path.join(get_package_share_directory('vrx_gazebo'), 'models/' + model.model_name + '/tmp')
urdf_file = os.path.join(model_dir, 'model.urdf')
with open(urdf_file, 'r') as infp:
    robot_desc = infp.read()
params = {'use_sim_time': use_sim_time, 'frame_prefix': model.model_name + '/', 'robot_description': robot_desc}
nodes.append(Node(package='robot_state_publisher',
                      executable='robot_state_publisher',
                      output='both',
                      parameters=[params],
                      remappings=[('/joint_states', '/' + model.model_name + '/joint_states')]))