.Net Fix - Display thread type name in error message (#12723)

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Error message confusing

### Description
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Error message does not display thread type and instead displays the name
of the generic parameter:

`Microsoft.SemanticKernel.KernelException: OpenAIResponseAgent currently
only supports agent threads of type TThreadType.`

(Customer reported an error using the erroneous error message.)

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- [X] The code builds clean without any errors or warnings
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Author: crickman

dotnet/src/Agents/Abstractions/Agent.cs

@@ -350,7 +350,7 @@ protected virtual async Task<TThreadType> EnsureThreadExistsWithMessagesAsync<TT
 
         if (thread is not TThreadType concreteThreadType)
         {
-            throw new KernelException($"{this.GetType().Name} currently only supports agent threads of type {nameof(TThreadType)}.");
+            throw new KernelException($"{this.GetType().Name} currently only supports agent threads of type {typeof(TThreadType).Name}.");
         }
 
         // We have to explicitly call create here to ensure that the thread is created