.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