Fix Optional target not using static builder factory method - #4058

When the return type of a mapping method is Optional and T uses a
Lombok @builder, MapStruct was generating new T.TBuilder() instead of
T.builder(). This caused a compilation error when the mapper and T are
in different packages, because the generated builder constructor is
package-private.

Root cause: ObjectFactoryMethodResolver.getBuilderFactoryMethod(Method, BuilderType)
was passing method.getReturnType() (i.e. Optional) to the overload
that checks assignability. Since T is not assignable to Optional,
the check failed and returned null, causing the fallback to use the
constructor directly.

Fix: Unwrap the Optional type before passing to the overload.

Fixes #4046