.NET: Update GroupChat workflow builder to support name and description

[peibekwe]

Description

This PR updates the GroupChatWorkflowBuilder to support name and description on the workflow for better user experience using DevUI.

Fixes #1885

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.

[Copilot]

Pull request overview

This PR enhances the GroupChatWorkflowBuilder by adding support for setting a name and description on workflows, improving the developer experience when using DevUI.

Changes:

• Added WithName() and WithDescription() methods to GroupChatWorkflowBuilder
• Updated the Build() method to apply name and description to the workflow when provided
• Added comprehensive unit tests covering various scenarios for name and description settings

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 3 comments.

File	Description
GroupChatWorkflowBuilder.cs	Added private fields for name/description and new builder methods to set them; updated Build() to conditionally apply these values
AgentWorkflowBuilderTests.cs	Added three new unit tests to verify name/description behavior in different scenarios
Program.cs	Demonstrated usage by adding name and description to the round-robin translation workflow example

[Copilot]

Initializing _name and _description to string.Empty will prevent them from being null when not set, but the tests expect null values. Consider initializing these fields to null (private string? _name; private string? _description;) to match the expected behavior shown in the test BuildGroupChat_WithoutNameOrDescription_DefaultsToNull.

Suggested change

	private string _name = string.Empty;
	private string _description = string.Empty;
	private string? _name;
	private string? _description;

[Copilot]

The WithName method does not validate if the name parameter is null. Consider adding null validation or documenting the expected behavior when null is passed, especially since the field is initialized to string.Empty.

[Copilot]

The WithDescription method does not validate if the description parameter is null. Consider adding null validation or documenting the expected behavior when null is passed, especially since the field is initialized to string.Empty.

Suggested change

	{
	{
	Throw.IfNull(description);