DOTNET - AG-UI: events should include Agent Name for multi-agent workflows

[graemefoster]

Summary

In multi-agent workflows (e.g. GroupChat), the AG-UI SSE events does not include the agent name, making it impossible for clients to identify which agent produced each message.

Impact

The AG-UI protocol already supports a name field on messages (BaseMessage.name), but multi-agent clients cannot identify which agent is speaking during streaming. This affects:

• GroupChat workflows — the client sees multiple TEXT_MESSAGE_START events from different agents in a single run but cannot label them
• Handoff workflows — same issue when control passes between agents
• Any UI that wants to show "Agent X is responding..." during streaming

Environment

• Microsoft.Agents.AI.Hosting.AGUI.AspNetCore 1.0.0-preview.260311.1
• Microsoft.Agents.AI.Workflows 1.0.0-rc4
• .NET 10.0
• GroupChat workflow with custom GroupChatManager