Contact cleanup reliability when changing physics engines in Gazebo 11

Goal: verify that contact memory created during short‑lived collisions is released when a model is spawned and then removed through Gazebo’s world/model lifecycle interface, independent of how the model was added.

Constraints: Gazebo forwards collision detection to interchangeable physics engines (ODE, Bullet, DART, Simbody), so ownership of contact objects is engine‑dependent; the cleanup path may vary between Gazebo Classic and the Gazebo/Ignition line, and the documentation does not explicitly state that all engines discard contact data on model deletion.

Questions: Does Gazebo 11 with ODE guarantee deletion of all contact allocations after a model is removed? Does the same guarantee hold when using Bullet or DART? Is the behavior consistent across Gazebo Classic and Gazebo/Ignition releases for a given engine?