.NET: fix: AIAgent - DebuggerDisplay using a non-existing Property

[rwjdk]

Motivation and Context

In a recent update, the property DisplayName was removed from AIAgent.cs, but the property is still used in the class DebuggerDisplay attribute.

Because of this, when debugging, you see this when hovering the mouse over the agent:

Description

This PR fixes this by introducing a private DebuggerDisplay (like many other classes in AF do) that either shows Name + Id (if name is provided)

or just the Id if the Agent does not have a name

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.

[rwjdk]

Can see this is a duplicate of #2985 so properly not needed