How can I use rospy through PyCall directly?

The code I have can only be able to run the callback once.

```julia
using PyCall

function solve_callback(data)
println("solve_callback.")
end

rospy = pyimport("rospy")
msg = pyimport("geometry_msgs.msg")
WrenchStamped = msg.WrenchStamped

rospy.init_node("solver_interface_sub", disable_signals = false)
println("init_node solver_interface_sub.")

solver_sub = rospy.Subscriber("/solver_input", WrenchStamped, solve_callback, queue_size=1)

if rospy.is_shutdown()
@warn "rospy.is_shutdown()"
end

rospy.spin()

println("stop.")

```
I am aware that RobotOS.jl is one method to accomplish this, but I want to know how can I call rospy directly using PyCall.

Thank you very much.

Start with the PyCall callback and rospy.Subscriber/rospy.spin calls shown in the report, and compare them with RobotOS.jl's handling of ROS callbacks. Reproduce the one-callback behavior using the supplied Julia snippet and investigate the callback lifetime or threading behavior involved. Done means a verified direct-PyCall approach that invokes solve_callback repeatedly, or a clear explanation of why it cannot.

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