Refactor default value handling logic

Updated `GetDefault` methods in `AutoObservableProvider.cs` and `AutoTaskProvider.cs` to use the `DefaultForType` class for determining default values, improving code modularity and reusability.

Author: Notallthatevil

src/NSubstitute/Routing/AutoValues/AutoObservableProvider.cs

@@ -21,6 +21,7 @@ public bool CanProvideValueFor(Type type) =>
 
     private static object? GetDefault(Type type)
     {
-        return type.GetTypeInfo().IsValueType ? Activator.CreateInstance(type) : null;
+        var defaultForType = new DefaultForType();
+        return defaultForType.GetDefaultFor(type);
     }
 }

src/NSubstitute/Routing/AutoValues/AutoTaskProvider.cs

@@ -1,3 +1,4 @@
+using NSubstitute.Core;
 using System.Reflection;
 
 namespace NSubstitute.Routing.AutoValues;
@@ -32,6 +33,7 @@ public object GetValue(Type type)
 
     private static object? GetDefault(Type type)
     {
-        return type.GetTypeInfo().IsValueType ? Activator.CreateInstance(type) : null;
+        var defaultForType = new DefaultForType();
+        return defaultForType.GetDefaultFor(type);
     }
 }