CrewAI Sample Failing

### What happened?

```
return session_data[image_key]
~~~~~~~~~~~~^^^^^^^^^^^
TypeError: 'NoneType' object is not subscriptable
ERROR:a2a.server.apps.jsonrpc.jsonrpc_app:Request Error (ID: ce21095b-602b-4455-b171-bdfe0c3ea043): Code=-32603, Message=''NoneType' object is not subscriptable'
```

Getting this error when testing the CrewAI Sample

### Relevant log output

```shell

```

### Code of Conduct

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

The error occurs in the CrewAI sample, likely in a2a/server/apps/jsonrpc/jsonrpc_app.py where session_data[image_key] is accessed. First, locate the CrewAI sample code and run it to reproduce the error. Examine the session_data variable to understand why it is None. Check the sample's setup and the JSON-RPC request flow to see what triggers this condition.

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