Function encode_payload_for_event 

pub fn encode_payload_for_event(payload: &[u8]) -> (PayloadEncoding, String)

Choose how to show an opaque payload to the model, and say which was chosen.

Text that is genuinely readable is shown as text; anything else is hex. The choice is reported in the event as payload_encoding, so the model can echo the same value back in its action and the round trip is symmetric.