[GDScript] Fix get_argument_count for lambda Callables

[AThousandShips]

• Fixes: get_argument_count() on Callable class reporting nested lamda calls as additional arguments. #93952

[AThousandShips]

Will check the difference with self and non-self callables and check both for completeness, will update in a bit when I've confirmed the cases

Edit: Done

[dalexeev]

The tests are overcomplicated, you can simply use:

var a: int
var lambda := func (x: int) -> void:
    print(a, x)
print(lambda.get_argument_count()) # Should print 1, prints 2 in master.

[AThousandShips]

Sure, can remove the extra cases, but will keep separate cases for self/non-self to get full coverage

Edit: will redo the test one moment

[akien-mga]

Thanks!

[AThousandShips]

Thank you!