.NET: Fix DebuggerDisplay attribute in AIAgent.cs to reference existing properties

[Copilot]

Motivation and Context

AIAgent.cs has a [DebuggerDisplay("{DisplayName,nq}")] attribute referencing a deleted DisplayName property, causing CS0103 errors during debugging.

Description

• Replaced DisplayName reference with a private DebuggerDisplay property
• Property shows Id always, plus Name when set
• Follows existing pattern used by InMemoryAgentThread and ChatClientAgentThread

[DebuggerDisplay("{DebuggerDisplay,nq}")]
public abstract class AIAgent
{
    [DebuggerBrowsable(DebuggerBrowsableState.Never)]
    private string DebuggerDisplay =>
        this.Name is { } name ? $"Id = {this.Id}, Name = {name}" : $"Id = {this.Id}";

Contribution Checklist

• The code builds clean without any errors or warnings
• The PR follows the Contribution Guidelines
• All unit tests pass, and I have added new tests where possible
• Is this a breaking change? If yes, add "[BREAKING]" prefix to the title of the PR.

Original prompt

---

This section details on the original issue you should resolve

<issue_title>.NET: AIAgent.cs refer to deleted DisplayName in DebuggerDisplay</issue_title>
<issue_description>Small one...

In AIAgent.cs, the DebuggerDisplay still refers to the now deleted DisplayName

Causing "error" during debugging

</issue_description>

<agent_instructions>Remove DisplayName from the debugger attribute. Add a private debugger display property that includes the ID as well as name if it's set. </agent_instructions>

Comments on the Issue (you are @copilot in this section)

• Fixes .NET: AIAgent.cs refer to deleted DisplayName in DebuggerDisplay #2983

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[Copilot]

Pull request overview

This PR fixes a broken DebuggerDisplay attribute in AIAgent.cs that was referencing a deleted DisplayName property, causing CS0103 errors during debugging.

• Replaced broken DisplayName reference with a working DebuggerDisplay private property
• The new property displays Id always, plus Name when it's set
• Follows existing debugger display patterns used in InMemoryAgentThread and ChatClientAgentThread