net: lwm2m: Make sure Sensor Type string isn't too long

The default string representing Sensor Type resource in Generic IPSO
object would not fit into the predefined buffer. Increase the buffer
size and add extra BUILD_ASSERT to detect this situation.

Coverity ID: 214225

Fixes #28164

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>