Fix: add pub to the test struct GdSelfObj

[ColinWttt]

Private GdSelfObj cause signal test failed

[Yarwin]

https://github.com/Yarwin/gdext/actions/runs/13603957943

Which test? In which CI job? How does changing visibility helps with this issue?

I am unable to replicate this neither in the CI nor locally.

[Bromeon]

Background: moving GdSelfObj into its own module causes problems because the generated signal methods signals().xy() are no longer accessible.

I created the WIP branch bugfix/signal-visibility trying to address this, but there are some hard problems:

1. The signal generating proc macro #[godot_api] doesn't know about the visibility of the class struct, since that one is handled by a different proc macro #[derive(GodotClass)].
2. The xy() methods cannot be "more public" than the surrounding class, because:
   • Each such method returns a distinct type __godot_Signal_C_Xy
   • Each such type has a Deref to TypedSignal<'c, C, Ps>
   • The C in TypedSignal is the surrounding class
   • If xy() is public, then __godot_Signal_C_Xy must be public, and all generic args of TypedSignal<'c, C, Ps> must be public
3. If C (surrounding class) is private, the code won't compile due to "cannot leak private type in public function" style errors.

Adding a pub in the test just works around the problem, it still won't work for private types. As such, this PR unfortunately doesn't solve anything.

We need to address this differently -- maybe there's a neat trick with modules or so, but it might also be that users would have to explicitly specify the visibility in #[godot_api].