Function process_stream_event 

pub fn process_stream_event(
    event: StreamEvent,
    messages: &mut Vec<Value>,
    subagents: &mut Vec<SubagentTracker>,
    queued_message: &mut Option<String>,
    pending_events: &mut Vec<String>,
) -> (EngineStreamEvent, StreamCompletion)

Convert a raw StreamEvent into an EngineStreamEvent. Also handles message history capture and returns completion signals.

messages — the conversation history (updated in place on MessageHistory) subagents — tracked subagent states (updated in place) queued_message — message queued during streaming (taken if stream completes) pending_events — events buffered during streaming (drained on completion)