I think some people are getting hung up on issues tangential to the question, such as the ternary operator. Yes, lots of people hate it, so maybe it's good to bring up anyway.
Concerning the focus of your question, moving the returned statement out to be referenced by a variable...
This question makes 2 assumptions that I disagree with:
That the second variant is more clear or easy to read (I say the opposite is true), and
that everyone uses Visual Studio. I have used Visual Studio many times and can use it just fine, but I usually am using something else. A dev environment that forces a specific IDE is one I would be skeptical of.
Breaking something out to a named variable rarely ever makes anything harder to read, it almost always does the opposite. The specific manner in which someone does it could cause problems, like if a self-documentation overlord does
var thisVariableIsTheFormattedResultAndWillBeTheReturnValue = ... then obviously that is bad, but that is a separate issue.
var formattedText = ... is fine.
In this specific case, and possibly many cases since we are talking about 1-liners, the variable would not tell you much that the function name does not already tell you. Therefore, the variable does not add as much. The debugging argument could still hold, but again, in this specific case I don't see anything that is likely to be your focus when debugging, and it can always be easily changed later if somehow someone needs that format for debugging or anything else.
In general, and you did ask for the general rule (your example was just that, an example of a generalized form), all the points made in favor of variant 1 (2-liner) are correct. Those are good guidelines to have. But guidelines need to be flexible. For example, the project I'm working on now has an 80 character per line maximum, so I split a lot of lines, but I commonly find lines 81-85 characters which would be awkward to split or reduce readability and I leave them over the limit.
Since it's unlikely to add value, I would not do 2 lines for the specific example given. I would do variant 2 (the 1-liner) because the points are not strong enough to do otherwise in this case.