Update text handling and CI for Godot 4.5

Godot 4.5 introduced changes to string handling with regards to the NUL character, which we use a lot with InSim; this MR fixes this issue.

Additionally, now that GdUnit4 supports variadic functions, or at least doesn't immediately crash anymore, we will go ahead and attempt to move the CI pipeline to Godot 4.5, even though it isn't officially supported yet (it should only be once GdUnit version 6.0 is released, which is partially tracked by this GitHub issue).