feat: publish collision string message during emergency stop (Resolves #159)

[yenode]

New feature implementation

Implemented feature

This PR introduces a ROS 2 publisher to broadcast collision events, enabling automated CI and QA scripts to detect when simulated robots enter an emergency stop state.
Resolves: #159

Implementation description

To allow external testing scripts (such as those needed for rmf_ros2#504) to detect simulation level collisions, the following approach was taken:

In slotcar_common.cpp, modified the emergency_stop() method. When need_to_stop evaluates to true (indicating the robot is too close to an obstacle based on stop_distance and stop_radius), the node publishes a string containing the _model_name. This ensures test scripts can identify exactly which robot triggered the collision state.

GenAI Use

We follow OSRA's policy on GenAI tools

• I used a GenAI tool in this PR.
• I did not use GenAI

Generated-by: Gemini AI (Used exclusively for drafting and formatting this PR description text. All C++ code was written manually).

[arjo129]

This is great start. I'm wondering if we should add a custom message.

Also, we should find a way to stop gazebo and terminate rmf in such instances.

@luca-della-vedova, would love your input on this as well.

[arjo129]

Thanks for this PR! Do you think that we should use string or some other custom message?

[arjo129]

Also can you follow the instructions for DCO: https://github.com/open-rmf/rmf_simulation/pull/160/checks?check_run_id=67082126337

[yenode]

Thanks for the review and the feedback!

I agree with moving to a custom message.

Publishing the current _model_name, the timestamp, and the [x, y, z] coordinates should be more than enough for the CI script to fail the test and log the location of the collision.

Can I define a new RobotCollision.msg in the rmf_fleet_msgs package?

Regarding terminating Gazebo and RMF: Once the collision is published, do you prefer we attempt a ROS 2 lifecycle shutdown from within the simulation plugin itself, or should the Python CI test script listen for the custom message and broadcast a SIGINT to terminate the entire launch tree?