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/rollout/src/persistence_metrics_tests.rs

@@ -51,6 +51,7 @@ fn turn_started(turn_id: &str) -> RolloutItem {
 fn turn_complete(turn_id: &str) -> RolloutItem {
     RolloutItem::EventMsg(EventMsg::TurnComplete(TurnCompleteEvent {
         turn_id: turn_id.to_string(),
+        started_at: None,
         last_agent_message: None,
         completed_at: None,
         duration_ms: None,
@@ -61,6 +62,7 @@ fn turn_complete(turn_id: &str) -> RolloutItem {
 fn turn_aborted(turn_id: &str) -> RolloutItem {
     RolloutItem::EventMsg(EventMsg::TurnAborted(TurnAbortedEvent {
         turn_id: Some(turn_id.to_string()),
+        started_at: None,
         reason: TurnAbortReason::Interrupted,
         completed_at: None,
         duration_ms: None,