Extracts user message content from an ACP protocol timeline event.
Returns the extracted content when the event represents a user-sent
prompt (session/prompt with origin: "USER_EVENT").
Returns null for all other events.
Expects the parsed payload produced by classifyACPAxonEvent,
where data is the params-level object ({ prompt, sessionId }).
Extracts user message content from an ACP protocol timeline event.
Returns the extracted content when the event represents a user-sent prompt (
session/promptwithorigin: "USER_EVENT"). Returnsnullfor all other events.Expects the parsed payload produced by classifyACPAxonEvent, where
datais the params-level object ({ prompt, sessionId }).