.NET: Preserve and propagate CreatedAt through workflows (#3930)

* Preserve per-message CreatedAt attribute if it's available

* Add unit test

---------

Co-authored-by: Sam Chang <changsam@microsoft.com>
Co-authored-by: samchang-msft <samchang.msft@gmail.com>

dotnet/src/Microsoft.Agents.AI.Abstractions/AgentResponse.cs

@@ -297,7 +297,7 @@ public class AgentResponse
                 AgentId = this.AgentId,
                 ResponseId = this.ResponseId,
                 MessageId = message.MessageId,
-                CreatedAt = this.CreatedAt,
+                CreatedAt = message.CreatedAt ?? this.CreatedAt,
             };
         }