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/tasks/mod.rs

@@ -740,6 +740,7 @@ impl Session {
             turn_context.config.memories.use_memories,
             turn_had_memory_citation,
         );
+        let started_at = turn_context.turn_timing_state.started_at_unix_secs().await;
         let (completed_at, duration_ms) = turn_context
             .turn_timing_state
             .completed_at_and_duration_ms()
@@ -756,6 +757,7 @@ impl Session {
             EventMsg::TurnAborted(TurnAbortedEvent {
                 turn_id: Some(turn_context.sub_id.clone()),
                 reason,
+                started_at,
                 completed_at,
                 duration_ms,
             })
@@ -769,6 +771,7 @@ impl Session {
             EventMsg::TurnComplete(TurnCompleteEvent {
                 turn_id: turn_context.sub_id.clone(),
                 last_agent_message,
+                started_at,
                 completed_at,
                 duration_ms,
                 time_to_first_token_ms,
@@ -877,6 +880,11 @@ impl Session {
             }
         }
 
+        let started_at = task
+            .turn_context
+            .turn_timing_state
+            .started_at_unix_secs()
+            .await;
         let (completed_at, duration_ms) = task
             .turn_context
             .turn_timing_state
@@ -891,6 +899,7 @@ impl Session {
         let event = EventMsg::TurnAborted(TurnAbortedEvent {
             turn_id: Some(task.turn_context.sub_id.clone()),
             reason,
+            started_at,
             completed_at,
             duration_ms,
         });