Ask “what edge cases are there?” when first describing the task and let them go off and work a while.

Later ask “are there any other edge cases?” and let them work a while longer.

Repeat as appropriate.

Let them find them on their own for a while or two.

Let them miss them on their own for a while or two.

At the right time ask “have you thought about this potential edge case?”

If you want someone to learn to deal with edge cases, they need to see it is something you value and give them time to learn how to do it.

And how not to do it. “Not” in the logical sense not “not” in a punishment sense or job evaluation sense.

Edge cases are hard and everyone screws them up. The context needs to be safe enough that people can own their mistakes and learn something other than to hide their ignorance.

Good luck.