.Net: A2A Samples

[markwallace-microsoft]

1. Sample showing how communicate between two SK Chat Completion agents
   • Python
   • .Net
2. Sample showing how communicate between SK Agents and agents that implement A2A
   • Python
   • .Net

Reference: https://github.com/moonbox3/semantic-kernel/blob/a2a-with-sk/docs/decisions/00XX-A2A-integration-within-SK.md

[markwallace-microsoft]

Duplicate of #11493

[hansmbakker]

This is not a duplicate, because this issue is about the .NET version. The Python version might be done but the .NET version is still being worked on in #12050.

@markwallace-microsoft @moonbox3 is it an idea to reopen this, so that it can be tracked?