Support for A2A communication with Foundry Agents

I would like to configure the SDK to recognize and use Azure Foundry hosted agents as custom subagents instead of Github-hosted agents. Is this possible or what is the best way to think about doing this today?

Use case: I have defined a custom agent with Github Copilot SDK (C#) which behaves similarly to what plan mode does. I've setup this agent as a Foundry Hosted Agent service and would like the agent to leverage Foundry-based agents to execute and perform the work described in the plan (for security reasons, traceability, etc.) instead of proper GH subagents.

In this scenario, I would still like to see GHCP share memory/context with a Foundry agent in a similar manner as GHCP does with subagent today if possible.

[Foundry has A2A support](https://learn.microsoft.com/en-us/azure/foundry/agents/how-to/tools/agent-to-agent?tabs=prompt-agents&pivots=csharp) for its agents in preview. Does the SDK provide a mechanism to leverage other agent runtimes hosted on non-Github infrastructure?

该 issue 未指出任何文件、测试或入口点。首先定位 SDK 当前如何配置自定义子代理以及如何共享内存或上下文，然后将这些能力与 Azure Foundry 的 preview A2A 支持进行比较。完成标准是确定一致认可的集成路径，并记录调用 Foundry-hosted agents 时的行为。

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