Tuya climate: support "always on"

**Describe the problem you have/What new integration you would like**

Tuya climate wants a `switch_datapoint` to be able to control the thermostat on/off and show the mode & active state; if you don't specify any `switch_datapoint` it always reports "Off" for mode and active state.
It would be nice if the `switch_datapoint` would be optional with some kind of "always on" option, instead of "always off", which it is now. (the documentation says it's required, but in code it's optional, and effectively mode and state are "always off" if you don't specify it)

**Please describe your use case for this integration and alternatives you've tried:**

I have a temperature controller ([Inkbird ITC-308WIFI](https://inkbird.com/products/itc-308-wifi)) that has no on/off switch, it's always on when it is plugged to power. So also tuya doesn't report a datapoint for turning it on or off.
As a workaround, I now use an enum datapoint that never becomes 0, so effectively making it always on (and trying to set the thermostat to off just doesn't do anything except an error in esphome log), but it's a bit of a hack

**Additional context**

The request names no files or tests. Start by locating the Tuya climate integration and its switch_datapoint configuration, then trace how missing switch data affects reported mode and active state. Done means an always-on controller is represented correctly without requiring a switch datapoint, with the relevant behavior covered by tests or documented verification.

Written by the indexing model from the issue text.