On the cheap side, I would measure the voltage that powers the light and wire a mechanical relay in parallel to match the voltage. From there, use a contact sensor that accepts an external connection, such as the Eclolink door sensor.
The contact will then report the open/close status to SmartThings.
Here is a basic relay. You would simply wire the input to the light and output to the contact sensor. You can either configure it to be normally open or normally closed. Doesn’t really matter.