a2a client如何从a2a Serever 提供的多个Agent中选择某个agent？

在[a2a-python](https://github.com/google/a2a-python/tree/main)
/examples/下的helloworld和anggraph示例中，test_client.py客户端中调用Agent都是通过以下方式调用的：
client = await A2AClient.get_client_from_agent_card_url(
httpx_client, AGENT_URL
)
而此处的AGENT_URL是写死的，这样只能调用已知的Agent。我理解的是a2a server应该包含多个Agent，而a2a client 可以从Agent Server获取这些Agent的Agent Card，然后让a2a client 根据Agent Card的信息结合自己的需求调用相应的Agent。逻辑是这样的吗？目前实现这种逻辑了吗？有示例吗？如果没有，建议给出相应的示例。如果有，请帮忙给出相应的示例或者示例链接。多谢！

Examine the a2a-python repository's examples, particularly the test_client.py files, to understand how A2AClient currently works. Look for methods to list or discover agents from an A2A server. Check if there are existing agent card retrieval or selection mechanisms. The goal is to document or create an example showing client-side agent selection from multiple available agents.

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