Make Python EventEncoder robust to non-JSONable fields (pluggable serializer / pre-serialize hook)

[GinoLlerena]

When streaming AG-UI events from a LangGraph/LangChain backend, the Python encoder crashes if any non-JSON-serializable runtime object (e.g., langchain_core.callbacks.manager.AsyncCallbackManager) leaks into the event payload. The crash originates from the encoder calling event.model_dump_json(...), which is strict and fails on unknown types.