[Bug] Inconsistency happens when CSharpType is created from System.Nullable<T>

[ArcturusZhang]

Describe the issue or request

We decide to hide the System.Nullable<T> layer in our type system, for instance, we will not generate model a nullable int type as CSharpType { IsFrameworkType: true, FrameworkType: typeof(Nullable<>), Arguments: [ typeof(int) ]
Instead, we do this:

CSharpType { IsFrameworkType: true, FrameworkType: typeof(int), IsNullable: true }

But when we create an instance of CSharpType from typeof(int?), we still get an instance as a generic type of System.Nullable<T>.
The issue exists in this implementation is that if we do this:

CSharpType type = typeof(int?);
Assert.IsTrue(type.IsNullable); // this will fail.

This creates an inconsistency, and when we are creating types, we have to notice that we should not do the above, instead, we should do:

CSharpType type = new CSharpType(typeof(int), true); // this creates a nullable int
Assert.IsTrue(type.IsNullable); // this succeeds.

Unfortunately this is not covered by any test cases. We should add more to validate such an important infrastructure is working as expected.