GDExtension: Fix setting base class properties on a runtime class

[dsnopek]

Fixes #93676

This PR makes it so that placeholders for runtime classes from GDExtension will only try to store properties that are specifically from the runtime class (or any of its parent classes that are also runtime classes).

This fixes #93676 in my testing, but also fixes other issues that I hadn't noticed previously with setting properties on parent classes. For example, before this PR, if you make a runtime class in a GDExtension that extends Sprite, then set the texture property in the editor, it won't actually display the texture in the editor (but will when loading the saved scene in the running game).

Also, while working on this, I realized that we have a difference of behavior between runtime classes defined in modules and those defined in GDExtensions: in a GDExtension, if you make a runtime class that has a non-runtime parent class, it won't run any of the code from that parent class, but only use its native parent; whereas in a module, the parent is always native, and it would always run the parent's code. So, if someone used runtime classes that extended non-runtime classes from their GDExtension, and then moved those classes into a module, they would act differently.

I think it should be possible to make GDExtensions work the same as modules in this regard, and I started hacking on it, but it would be a big patch. So, instead, I've added a new limitation preventing runtime classes defined in GDExtensions from extending from non-runtime extension classes. This way, when we eventually make the changes to correct this behavior difference, we can also remove this limitation, preventing a future change in behavior.

[akien-mga]

I tested and confirmed this fixes the Rust MRP.

[akien-mga]

Thanks!