.NET: A2A agent (#520)
* add a2a agent
* Update dotnet/src/Microsoft.Extensions.AI.Agents.A2A/A2AAgent.cs
Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>
* address pr review feedback
* move unit tests for extension methods to the extensions folder
* address pr review comments
* address pr review comments
* address pr review feedback
* move a2a agent sample to console app
* remove unnecessary Ids set for new projects in the solution file
* remove unnecessary configuration
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Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>
SergeyMenshykh