Preserve item timestamps in thread history projections (#35689)

## What changed

- Add optional start and completion timestamps to `ThreadHistoryItemChange`.
- Populate them from canonical `ItemCompleted` records during stateless thread history projection.
- Treat a zero completion timestamp as absent, and leave timestamps unset for legacy builder events that do not provide them.

## Testing

- Cover projected item timestamps, including missing start times and zero completion times.

GitOrigin-RevId: 2187530f489ad3d86b6ab65faaaab1eab5357f1b

codex-rs/app-server-protocol/src/protocol/thread_history_projection.rs

@@ -70,6 +70,8 @@ pub fn project_rollout_line(line: &RolloutLine) -> ThreadHistoryChangeSet {
             changed_items: vec![ThreadHistoryItemChange {
                 turn_id: event.turn_id.clone(),
                 item: ThreadItem::from(event.item.clone()),
+                started_at_ms: event.started_at_ms,
+                completed_at_ms: (event.completed_at_ms != 0).then_some(event.completed_at_ms),
             }],
             ..Default::default()
         },