There isn’t one.
There’s a new zigbee fan controller available from Home Depot that some community members are trying. I’m not sure if it has a wall switch, though, it might just have a handheld remote.
Otherwise if you want to go zwave, you need two devices, One for the light and one for the fan speed control. Some people use two switches side-by-side and a double gang box, some put one switch on the wall and put a micro in the fan canopy and make one of the options changeable only through a tablet or phone app. Or minimote. This gives you a full integration, really the only downside is that it’s not a single gang wall device.
Or
Lutron does have a fan controller which has light and fan speed in a single gang device, but it’s not from the Caseta line. To get integration with SmartThings you need to also have a harmony hub in the room with the switch. And it’s a one-way integration, which means if someone makes a change at the wall switch, SmartThings won’t know about it. So not a very satisfying solution, but it does work for some people.
https://www.amazon.com/Lutron-MIR-LFQMT-WH-Maestro-Dimmer-Control/dp/B0017OAWQ0