GDExtension: Call startup callback only after reload is fully finished

[dsnopek]

It was brought up on chat.godotengine.org by @Yarwin that we're currently calling a GDExtension's startup callback before the extension has been fully reloaded, when any objects that use its classes are in an inconsistent state.

Instead, we'd want the startup callback to be called after reload is fully finished, so that the state is roughly the same as if the extension was being loaded for the first time.

This PR attempts to do that, by moving the "after load" stuff into a _finish_load_extension() method, which won't be called automatically on reload, so that the reload process can call it at the end

This startup callback was functionality added for Godot 4.5, it'd be good to get the order correct before release

[Ivorforce]

Looks good to me.
Just an implementation thought; since _load_extension_internal is called with p_first_load=true only from load_extension_with_loader, maybe the _finish_load_extension should be called from load_extension_with_loader instead?

[dsnopek]

@Ivorforce Thanks!

Just an implementation thought; since _load_extension_internal is called with p_first_load=true only from load_extension_with_loader, maybe the _finish_load_extension should be called from load_extension_with_loader instead?

Yeah, I had thought about that. My thinking was that I wanted to make it clear that _finish_load_extension() should follow _load_extension_internal(). But since this is all internal stuff, and it's all very close together, it's probably clear regardless of how it's done.

In my latest push, I've switched to _finish_load_extension() being called "manually" after _load_extension_internal() in both cases

[bruvzg]

Looks good.

It's internal method so doesn't ready matter, but maybe it should be called something like notify_extension_loaded or similar, since it's only sending signal / callback after actual (re)loading is done.

[Repiteo]

Thanks!