.NET: [Breaking] Delete display name property (#2758)

* delete the AIAgent.DisplayName property

* use agent name as a first value for activity display name

* Update dotnet/src/Microsoft.Agents.AI.Workflows/Specialized/HandoffAgentExecutor.cs

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

---------

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

dotnet/tests/Microsoft.Agents.AI.Workflows.UnitTests/Sample/12_HandOff_HostAsAgent.cs

@@ -30,7 +30,7 @@ internal sealed class HandoffTestEchoAgent(string id, string name, string prefix
                 {
                     return [new(ChatRole.Assistant, [new FunctionCallContent(Guid.NewGuid().ToString("N"), handoff.Name)])
                     {
-                        AuthorName = this.DisplayName,
+                        AuthorName = this.Name ?? this.Id,
                         MessageId = Guid.NewGuid().ToString("N"),
                         CreatedAt = DateTime.UtcNow
                     }];