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/tools/handlers/multi_agents_tests.rs

@@ -1558,6 +1558,7 @@ async fn multi_agent_v2_list_agents_returns_completed_status_without_encrypted_s
             child_turn.as_ref(),
             EventMsg::TurnComplete(TurnCompleteEvent {
                 turn_id: child_turn.sub_id.clone(),
+                started_at: None,
                 last_agent_message: Some("done".to_string()),
                 completed_at: None,
                 duration_ms: None,
@@ -2007,6 +2008,7 @@ async fn multi_agent_v2_followup_task_completion_notifies_parent_on_every_turn()
             first_turn.as_ref(),
             EventMsg::TurnComplete(TurnCompleteEvent {
                 turn_id: first_turn.sub_id.clone(),
+                started_at: None,
                 last_agent_message: Some("first done".to_string()),
                 completed_at: None,
                 duration_ms: None,
@@ -2048,6 +2050,7 @@ async fn multi_agent_v2_followup_task_completion_notifies_parent_on_every_turn()
             second_turn.as_ref(),
             EventMsg::TurnComplete(TurnCompleteEvent {
                 turn_id: second_turn.sub_id.clone(),
+                started_at: None,
                 last_agent_message: Some("second done".to_string()),
                 completed_at: None,
                 duration_ms: None,
@@ -2210,6 +2213,7 @@ async fn multi_agent_v2_interrupted_turn_does_not_notify_parent() {
             aborted_turn.as_ref(),
             EventMsg::TurnAborted(TurnAbortedEvent {
                 turn_id: Some(aborted_turn.sub_id.clone()),
+                started_at: None,
                 reason: TurnAbortReason::Interrupted,
                 completed_at: None,
                 duration_ms: None,