Java A2A Agents cannot be accessed Using Python ADK RemoteA2aAgent

### What happened?

I tested this with several Java agents such as the Weather Agent and Content Writer Agent. It appears that all Java agents exposed via A2A fail to return responses correctly when accessed through ADK RemoteA2aAgent.

Steps to Reproduce:

1) Run any Java agent (e.g., Content Editor Agent).

2) Create a simple agent using ADK:
`root_agent = RemoteA2aAgent(
name="remote_agent",
description="Helpful assistant to write content",
agent_card=f"http://localhost:10003/{AGENT_CARD_WELL_KNOWN_PATH}",
)
`

3) Run **adk web** , select the remote_agent and try to chat with the agent.
- The response does not appear in the UI.

### Relevant log output

```shell

```

### Code of Conduct

- [x] I agree to follow this project's Code of Conduct

The issue involves Java agents (like Weather Agent, Content Writer Agent) and the Python ADK's RemoteA2aAgent. Start by examining the Java agent's A2A server setup and the Python ADK client code for RemoteA2aAgent. Look at the agent card endpoint (http://localhost:10003/{AGENT_CARD_WELL_KNOWN_PATH}) and the communication protocol. Run the Java agent and the ADK web interface to reproduce the missing UI response. Check logs from both sides for errors in serialization or network communication. 'Done' is when the remote Java agent's responses appear correctly in the ADK web chat UI.

Written by the indexing model from the issue text.