Consider the case if the user does NOT have Flutter SDK installed at all

Let's test if the user has the GCLI extension installed, but does NOT have Flutter SDK installed.

Presumably, Gemini CLI will notice this, and ask the user if it should download it. In any case, let's test what happens.

There should be a reasonably good user experience here, which either tells the user that Flutter SDK is not installed and "go read the docs". or, try to install it.