[Fix] Cleanup Launch Files

[philipchurchley]

Addresses #950 and follows up on #959 by cleaning the remaining launch files. The launch files have been improved to use a declarative programming style, as described by @emersonknapp at ROSCon, which can be viewed in the livestream at 6:10.

Summary

• Converted ROS 2 Python launch files to a consistent declarative style (single
  LaunchDescription([...]), replaced PathJoinSubstitution with PathSubstitution + /,
  used literal "xacro" in Command, and removed RegisterEventHandler/OnProcessExit).

Validation performed

• Ran non-building smoke checks (import + generate_launch_description() for each launch) — all passed.
• Ran a semantic comparator vs master; it flagged 18 files but most differences are cosmetic or expected.

Potential confusions for reviewer attention

• example_15: added publisher_config launch argument in test launches. Please confirm the default file and behavior.
• Removal of RegisterEventHandler entries may affect startup/shutdown ordering for some bringup launches. Please check that this removal doesn't break anything.

Recommendation

• I have no experience with Ros2 so I’m not certain about runtime impacts of the items above. Please review flagged files and run integration/CI launch tests where ordering or parameters matter. If reviewers find real regressions, please recommend minimal fixes (re-introduce ordering, adjust defaults, or correct node fields).

[philipchurchley]

I will be away for the next week, so I apologize if I do not immediately respond to comments.

[christophfroehlich]

Jobs are failing, can you have a look please?