You're the engineer, not them - you're being paid to write code that ideally doesn't break - it's your job to explain that tests benefit them. There's very little excuse not to add test coverage as you go along these days given an LLM could help write most in a flash.
Or, don't tell them and do it anyway. It'll make your own life 100x easier, unless you enjoy the stress of bug fixing and responding to incidents where you've broken something.
Writing code without tests because it takes too long is like running a car engine without ever bothering to change the oil - it'll run for a bit, and eventually fail catastrophically.
