Persist realtime events in the thread timeline (#40508)

## Why

Clients need a bounded view of realtime conversations that preserves the order of speech, agent work, and turn lifecycle events without loading the full thread history.

## What changed

- Persist realtime session boundaries, transcript segments, and promoted agent items in paginated thread rollouts.
- Add the experimental `thread/timeline/list` API to page ordinary items, realtime items, and turn boundaries in canonical rollout order. Include `activeRealtimeSessionAtPageStart` so each page can be rendered independently.
- Emit typed `thread/realtime/item/started`, `thread/realtime/item/transcript/delta`, and `thread/realtime/item/completed` notifications around durable realtime items.
- Keep the existing `thread/items/list` response and legacy thread history behavior unchanged.

## Testing

- Cover mixed timeline pagination, realtime-session state at page boundaries, rollout replay, subagent history prefixes, transcript streaming, steering, and promoted agent artifacts.

GitOrigin-RevId: faa2c420192d63e060e8fb32446e3c797422656e

codex-rs/app-server/src/request_processors/thread_processor.rs

@@ -870,6 +870,35 @@ impl ThreadRequestProcessor {
             .map(|response| Some(response.into()))
     }
 
+    pub(crate) async fn thread_timeline_list(
+        &self,
+        params: ThreadTimelineListParams,
+    ) -> Result<Option<ClientResponsePayload>, JSONRPCErrorError> {
+        let thread_id = ThreadId::from_string(&params.thread_id)
+            .map_err(|error| invalid_request(format!("invalid thread id: {error}")))?;
+        let page = self
+            .thread_store
+            .list_timeline(StoreListTimelineParams {
+                thread_id,
+                cursor: params.cursor,
+                page_size: params
+                    .limit
+                    .map(|limit| limit as usize)
+                    .unwrap_or(THREAD_ITEMS_DEFAULT_LIMIT)
+                    .clamp(1, THREAD_ITEMS_MAX_LIMIT),
+            })
+            .await
+            .map_err(paginated_history_list_error)?;
+        Ok(Some(
+            ThreadTimelineListResponse {
+                data: page.items,
+                next_cursor: page.next_cursor,
+                active_realtime_session_at_page_start: page.active_realtime_session_at_page_start,
+            }
+            .into(),
+        ))
+    }
+
     pub(crate) async fn thread_shell_command(
         &self,
         request_id: &ConnectionRequestId,