You have to write your own AI code in every single general purpose game engine (that i know of atleast) on the market. I have a hard time even seeing how a "AI Editor" should work to be useful beyond a very specific game genre.
That's not true. Check out xaitment, for example: http://www.xaitment.com/
Incidentally xaitment has Unity support
xaitment is not a general purpose game engine though, it is a specialized plugin for one. (FSMs are not a general purpose game AI solution)
You can learn and even master C# using Unity3D if you want. that is entierly up to you. Unity won't encourage you to explore all parts of the language though, most tutorials and documentation are focused on extending behaviours). Also, since Unity3D uses mono you won't have immediate support for the latest C# version (mono lags a bit behind).