Enable RViz joint state publisher by default in vertical_chess launch

[juliaduerr]

Motivation

• RViz was showing a RobotModel status error due to missing joint state messages, so the launch should start a joint state publisher automatically to populate TF/joint data.
• Make the RViz joint state publisher optional but enabled by default so visualization works out-of-the-box while still allowing it to be disabled when not needed.

Description

• Add a new launch argument DeclareLaunchArgument("enable_rviz_joint_state_publisher", default_value="true", ...) to control whether the RViz joint state publisher runs.
• Add a Node entry for the ainex_controller package executable rviz_joint_state_publisher guarded by IfCondition(LaunchConfiguration("enable_rviz_joint_state_publisher")).
• Register an exit handler for the new node with create_exit_handler(...) and include the new argument, node, and exit handler in the returned LaunchDescription so it participates in the launch lifecycle.

Testing

• No automated tests were run because this is a configuration-only change; verification consists of launch file edits and a commit.

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Codex Task