.NET: [AG-UI] Setup infrastructure and enable text streaming

[javiercn]

Infrastructure: Implement the foundational infrastructure for AG-UI protocol support in the .NET Agent Framework, including:

• Two new packages: Microsoft.Agents.AI.AGUI for client-side consumption and Microsoft.Agents.AI.Hosting.AGUI.AspNetCore for server-side hosting
• Core event models for the AG-UI protocol with bidirectional conversion between AG-UI events and framework-native types (AgentRunResponseUpdate, ChatMessage)
• HTTP/SSE streaming support and thread management via AGUIAgentThread
• Server-side MapAGUIAgent extension method for exposing .NET agents as AG-UI endpoints
• Client-side AGUIAgent class for consuming any AG-UI-compliant server

Text Streaming: Implement Phase 1 text streaming capabilities with support for lifecycle events (RunStarted, RunFinished, RunError) and text message events (TextMessageStart, TextMessageContent, TextMessageEnd). Enable real-time streaming of conversational text between AG-UI clients and servers with proper Server-Sent Events (SSE) formatting. Include comprehensive testing (unit tests, integration tests for end-to-end scenarios) and a complete AGUIClientServer sample application demonstrating both client and server usage. Keep AG-UI protocol details internal while exposing clean public APIs aligned with the framework's abstractions.