Include start times in terminal turn events (#32263)

## What changed

- Add an optional `started_at` Unix timestamp to `TurnCompleteEvent` and
  `TurnAbortedEvent`.
- Populate the timestamp from turn timing state when a turn completes or is
  aborted.
- Preserve the timestamp when synthesizing interrupted fork history and when
  importing external sessions.

## Testing

- Verify that interrupting a turn emits a `TurnAbortedEvent` with a start
  timestamp.

GitOrigin-RevId: 75b3911b95c060e5fc1843a5e293d8fe47377186

codex-rs/core/src/codex_delegate_tests.rs

@@ -56,6 +56,7 @@ async fn forward_events_filters_private_events_before_blocked_send_is_cancelled(
             id: "full".to_string(),
             msg: EventMsg::TurnAborted(TurnAbortedEvent {
                 turn_id: Some("turn-1".to_string()),
+                started_at: None,
                 reason: TurnAbortReason::Interrupted,
                 completed_at: None,
                 duration_ms: None,