You cannot damage the engine by over-leaning, at the relatively low power settings involved in ground ops.
It is normal practice among advanced pilots to lean during taxi, to the maximum degree possible. It keeps the plugs clean, and you cannot takeoff with it because the engine would just cough and splutter if you went to full throttle like that.
In general, to cause detonation, you need the wrong mixture, a high power setting (a high combustion chamber pressure) and a very high CHT. Or the wrong grade of fuel (unsufficient octane rating, etc).