Include item start times in completion events (#35363)

## What changed

- Add an optional `started_at_ms` field to `ItemCompletedEvent`, retaining
  compatibility with older persisted events that lack it.
- Track the first start timestamp for each in-flight item and attach it when
  emitting and persisting the completion event. If no start was recorded, use
  the completion timestamp as a fallback.
- Emit a complete start/completion lifecycle for subagent activity items.

## Testing

- Cover concurrent item timing, repeated starts, turn-boundary cleanup, the
  missing-start fallback, subagent activity, and persisted web-search events.

GitOrigin-RevId: e7cec9c4f1ef6ba67f287e81fb4d7d856fcf87a7

codex-rs/thread-store/src/local/mod.rs

@@ -1580,6 +1580,7 @@ mod tests {
                 client_id: None,
                 content: Vec::new(),
             }),
+            started_at_ms: Some(0),
             completed_at_ms: 1,
         }));
         store