[Bug]: Agent card advertises internal URL instead of exposed URL

### What happened?

When running an A2A agent inside a container and publishing the container (e.g 8000) to a different host port (e.g. 8001), clients outside the container can resolve the agent card but then fail on subsequent calls. After doing some tests, I managed to identify that this is due to the agent card advertising the container's domain and port (e.g localhost:8000) instead of the host's (e.g mydomain:8001).

Apart from leaking container-internal addressing, this makes it very difficult to work with A2A agents when putting those inside a container or (I'm guessing) behind load balancers or proxies.

I'd be happy to provide an MRE if this behavior is indeed recognized as problematic or if my explanations aren't sufficient.

### Relevant log output

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