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/app-server/tests/suite/v2/thread_resume.rs

@@ -2268,6 +2268,7 @@ async fn thread_resume_token_usage_replay_can_belong_to_interrupted_turn() -> Re
             "type": "event_msg",
             "payload": serde_json::to_value(EventMsg::TurnAborted(TurnAbortedEvent {
                 turn_id: Some(interrupted_turn_id.to_string()),
+                started_at: None,
                 reason: TurnAbortReason::Interrupted,
                 completed_at: None,
                 duration_ms: None,