For many things where Godot requires strings, there are caches that can be used instead of typing strings by hand. Like SignalName or MethodName. But I haven’t found one for action names. Is it true that there is none?
There is nothing comparable to SignalName or MethodName for input actions built in. Although, it should be quite straightforward to implement on the project side. For instance, here is my own version of it. Keep it in mind, input maps can be modified at runtime. So those constants might not be valid, depending on how you are managing input maps in your game (and that is part of the rationale why such thing is not built in).