Keep Gazebo server running without GUI

[yeseorizi]

Summary

Keep the Gazebo server and entity spawning active when DAVE is launched without the GUI.

Problem

The top-level robot, sensor, and object launch files conditioned the entire Gazebo include on gui. As a result, gui:=false disabled not only the GUI client but also the simulation server. Spawn actions then waited for a world that did not exist.

Some child upload launch files also conditioned entity creation on headless, which prevented models from being inserted in valid server-only runs.

Changes

• Always include the Gazebo launch description.
• Run Gazebo in server-only mode when either headless:=true or gui:=false.
• Remove child conditions that prevented object and sensor creation in headless mode.
• Apply the same behavior consistently to robot, sensor, and object entry points.

Validation

Rebased onto ros2 after PR #65 and rebuilt the affected packages in an ARM64 Lyrical / Jetty container.

The following four cases passed:

1. Object: gui:=false headless:=false
2. Object: gui:=false headless:=true
3. DVL sensor: gui:=false headless:=true
4. RexROV robot: gui:=false headless:=true

Every case reported:

ready=true
server_count=1
gui_count=0
model_count=1

pre-commit run --all-files also passed.