.NET: Migrate AF A2AAgent and A2A Hosting to the latest A2A SDK

[SergeyMenshykh]

• Migration guide: https://github.com/a2aproject/a2a-dotnet/blob/main/docs/migration-guide-v1.md
• nuget: https://www.nuget.org/packages/A2A/1.0.0-preview