C#: Make value types assignable to their Nullable<T> counterparts

[knutwannheden]

Motivation

In C#, a value type T is implicitly convertible to Nullable<T> (T?). The TypeUtils.IsAssignableTo check did not account for this — IsAssignableTo(int, int?) returned false. This matters for C# recipes that need to reason about nullable assignability, such as the ThrowIfGreaterThan family of recipes where ThrowIfGreaterThan<T> has a where T : IComparable<T> constraint that Nullable<T> doesn't satisfy.

Examples

// These now return true:
TypeUtils.IsAssignableTo(intType, nullableIntType)       // int → int?
TypeUtils.IsAssignableTo(dateTimeType, nullableDateType)  // DateTime → DateTime?
TypeUtils.IsAssignableTo(nullableIntType, nullableIntType) // int? → int?

// These correctly return false:
TypeUtils.IsAssignableTo(intType, nullableDoubleType)     // int → double? (type mismatch)

Summary

• Handle Nullable<T> as a special case in IsAssignableTo(JavaType?, JavaType?) — when the target is Parameterized(System.Nullable, [T]), unwrap and check if the source is assignable to T
• Also handle Nullable<T> → Nullable<T> by unwrapping both sides
• Added 5 unit tests covering primitive, class, struct, mismatch, and self-assignment scenarios

Test plan

• All 60 TypeUtilsTests pass (including 5 new nullable tests)
• Existing tests unaffected (no behavioral change for non-nullable types)