Show launch argument names in print-description output

[official-burak]

Changelog

• Make --print-description show DeclareLaunchArgument names, defaults, and descriptions
• Identify OpaqueFunction by the wrapped callable
• Label LaunchDescription with its entity count

Description

Fixes #975

LaunchIntrospector.format_launch_description (what ros2 launch --print-description uses) fell through to Action('<... object at 0x...>') for most actions. Declared arguments therefore printed an address instead of the argument name.

Give those actions a useful __repr__ (Action.describe() already returns that) and use describe() in the introspector fallback.

Test evidence

Before:

<launch.launch_description.LaunchDescription object at 0x...>
└── Action('<launch.actions.declare_launch_argument.DeclareLaunchArgument object at 0x...>')

After:

LaunchDescription(1 entity)
└── DeclareLaunchArgument(name='use_sim_time', default_value='false', description='Use simulation clock')

pytest test/launch/test_launch_introspector.py test/launch/actions/test_declare_launch_argument.py failed first (missing use_sim_time / generate_nodes in the tree), then passed after the change (10 passed).