`convertFromVariant` can crash when in an invalid object

Found this when working with the indexing operator in Array. The old code for Array's indexed setter and indexed getter was this:

Variant operator [](int index) {
  final self = Variant(this);
  final ret = gde.variantGetIndexed(self, index);
  return convertFromVariant(ret, null) as Variant;
}

void operator []=(int index, Variant value) {
  final self = Variant(this);
  final variantValue = Variant(value);
  gde.variantSetIndexed(self, index, value);
}

Besides being inefficient, this was causing a crash because (I think) the engine was either mangling some pointers or the token was invalid when trying to convert to an Object.

I've taken the following steps to mitigate:

• Modified the index getter / setter in the code generation to avoid the extra Variant conversion when the return type is Variant.
• Avoid a null token when converting a variant to an Object by always passing in the TypeInfo for GodotObject
• Throw an exception if you try to construct a Variant with a Variant.

However, we should take further steps to ensure this isn't a bigger problem.

Start by tracing the generated Array indexed getter and setter, convertFromVariant, and Variant construction, including the GodotObject TypeInfo path. Reproduce conversion from an invalid object and verify that indexed access avoids the extra Variant conversion, rejects Variant-wrapping, and no longer crashes.