ROS 2 YAML launch: "choices" is not supported in arg

Description:

Using the "choices" key in an arg inside a ROS 2 YAML launch file causes a runtime error, but this limitation is not clearly documented.

Steps to reproduce:

1. Create YAML launch file:

launch:
  - arg:
      name: rviz
      default: "true"
      description: "Launch RViz2"
      choices: ["true", "false"]

2. Run:

ros2 launch <package> <file>.yaml

Observed behavior:

Launch fails with:

ValueError: Unexpected key(s) found in `arg`: {'choices'}

Expected behavior:

• Support "choices" in YAML launch files

Environment:

• ROS 2: Jazzy