Preserve item timestamps in thread history projections (#35689)

## What changed

- Add optional start and completion timestamps to `ThreadHistoryItemChange`.
- Populate them from canonical `ItemCompleted` records during stateless thread history projection.
- Treat a zero completion timestamp as absent, and leave timestamps unset for legacy builder events that do not provide them.

## Testing

- Cover projected item timestamps, including missing start times and zero completion times.

GitOrigin-RevId: 2187530f489ad3d86b6ab65faaaab1eab5357f1b

codex-rs/app-server-protocol/src/protocol/thread_history_projection_tests.rs

@@ -130,6 +130,8 @@ fn projects_completed_canonical_turn_items() {
         vec![ThreadHistoryItemChange {
             turn_id: "turn-1".to_string(),
             item: ThreadItem::from(user_item),
+            started_at_ms: Some(100),
+            completed_at_ms: Some(123),
         }]
     );
     assert_eq!(
@@ -137,10 +139,48 @@ fn projects_completed_canonical_turn_items() {
         vec![ThreadHistoryItemChange {
             turn_id: "turn-1".to_string(),
             item: ThreadItem::from(agent_item),
+            started_at_ms: Some(100),
+            completed_at_ms: Some(123),
         }]
     );
 }
 
+#[test]
+fn projects_optional_completed_item_lifecycle_timestamps() {
+    let item = TurnItem::UserMessage(UserMessageItem {
+        id: "user-1".to_string(),
+        client_id: None,
+        content: Vec::new(),
+    });
+
+    for (started_at_ms, completed_at_ms, expected_completed_at_ms) in
+        [(None, 123, Some(123)), (Some(100), 0, None)]
+    {
+        let changes = project(RolloutItem::EventMsg(EventMsg::ItemCompleted(
+            ItemCompletedEvent {
+                thread_id: ThreadId::default(),
+                turn_id: "turn-1".to_string(),
+                item: item.clone(),
+                started_at_ms,
+                completed_at_ms,
+            },
+        )));
+
+        assert_eq!(
+            changes,
+            ThreadHistoryChangeSet {
+                changed_items: vec![ThreadHistoryItemChange {
+                    turn_id: "turn-1".to_string(),
+                    item: ThreadItem::from(item.clone()),
+                    started_at_ms,
+                    completed_at_ms: expected_completed_at_ms,
+                }],
+                ..Default::default()
+            }
+        );
+    }
+}
+
 #[test]
 fn ignores_legacy_abort_without_turn_id_and_context_only_records() {
     let aborted = project(RolloutItem::EventMsg(EventMsg::TurnAborted(
@@ -206,7 +246,7 @@ fn item_completed(thread_id: ThreadId, turn_id: &str, item: TurnItem) -> Rollout
         thread_id,
         turn_id: turn_id.to_string(),
         item,
-        started_at_ms: Some(0),
+        started_at_ms: Some(100),
         completed_at_ms: 123,
     }))
 }