Core: Fix Object::has_method() for script static methods

[dalexeev]

• Fixes Callables of static methods are not valid if accessed via the class's name #79521.
• Supersedes Use custom callables for GDScript static methods #82755.

See #79521 (comment).

[RandomShaper]

My take on this very interesting issue:

My guts tell me that Script::has_method() belongs to a different concern that Object::has_method(). In other words, a script resource has methods appropriate to its nature (dealing with it as a script), so you can't say that a script happening to define some method for the object it will be attached to means that the script resource itself provides that method. In yet other words, Object::has_method() is intended to check the interface of the object itself, while Script::has_method() is intended to check the interface of the script inside, only that both happened to get the same name, unfortunately.

Then, there has to be a way for an object to include the interface defined by any attached script in its own, for purposes like calling, property setting/getting, and checking the interface for a method by name. This last point has motivated me to check the current implementation of Object::has_method(), to see if it actually includes whatever interface the attached script enrichens it with. For this discussion, these are the relevant lines:

	if (script_instance && script_instance->has_method(p_method)) {
		return true;
	}

So, script-wise it will only check the methods in the script if there's an instance. My first thought was to add code to find a static method in the attached script. But that's not right, because the object the Callable is based on, and so it does the validity check on it, is the class, the Script.

[RandomShaper]

Since I needed to test my hypothesis, I've done the changes and finally PRd them as #82770.

[akien-mga]

To corroborate RandomShaper's review, the compiler warning treated as error in this PR also exposes the same problem:

in core/object/script_language.h:
'has_method' overrides a member function but is not marked 'override' [-Werror,-Winconsistent-missing-override]

Script::has_method is virtual and it's the one that GDScript and CSharpScript override. It's not the same as Object::has_method, they just unfortunately share the same name (and this warning thankfully does it job at pointing out that this is creating an unexpected relationship).

[dalexeev]

Yes, I was confused that this method is called the same in Variant, Object and Script.

Here's another solution (virtual multi-level _has_method()): dalexeev@6a9944e. But I think that this is overkill, and it might conflict with GDExtension in the future, if we decide to expose the method.

[RandomShaper]

The current version of this PR includes the best of all the ideas discussed.

Just a note for the next compat breakage: rename Script::has_(_static)?method() to Script::has_script_(_static)?method() (I think the already existing Script::has_script_signal() and others are named like that to disambiguate).

[dsnopek]

The GDExtension related changes look good to me! Thanks :-)

[akien-mga]

Thanks!