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/src/bespoke_event_handling.rs

@@ -2191,6 +2191,7 @@ mod tests {
     fn turn_complete_event(turn_id: &str) -> TurnCompleteEvent {
         TurnCompleteEvent {
             turn_id: turn_id.to_string(),
+            started_at: None,
             last_agent_message: None,
             completed_at: Some(TEST_TURN_COMPLETED_AT),
             duration_ms: Some(TEST_TURN_DURATION_MS),
@@ -2201,6 +2202,7 @@ mod tests {
     fn turn_aborted_event(turn_id: &str) -> TurnAbortedEvent {
         TurnAbortedEvent {
             turn_id: Some(turn_id.to_string()),
+            started_at: None,
             reason: codex_protocol::protocol::TurnAbortReason::Interrupted,
             completed_at: Some(TEST_TURN_COMPLETED_AT),
             duration_ms: Some(TEST_TURN_DURATION_MS),