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/external-agent-migration/src/sessions/append.rs

@@ -217,7 +217,9 @@ fn source_model_items(items: &[RolloutItem]) -> Option<Vec<SourceModelItem<'_>>>
     let mut append_start_index = None;
     for (index, item) in items.iter().enumerate() {
         match item {
-            RolloutItem::SessionMeta(_) | RolloutItem::InterAgentCommunicationMetadata { .. } => {}
+            RolloutItem::SessionMeta(_)
+            | RolloutItem::InterAgentCommunicationMetadata { .. }
+            | RolloutItem::RealtimeItem(_) => {}
             RolloutItem::ResponseItem(response_item) => {
                 model_items.push(SourceModelItem {
                     response_item: &response_item.item,
@@ -255,6 +257,7 @@ fn history_model_items(items: &[RolloutItem]) -> Option<Vec<&ResponseItem>> {
         match item {
             RolloutItem::SessionMeta(_)
             | RolloutItem::InterAgentCommunicationMetadata { .. }
+            | RolloutItem::RealtimeItem(_)
             | RolloutItem::SecurityRiskScore(_) => {}
             RolloutItem::ResponseItem(response_item) => model_items.push(&response_item.item),
             RolloutItem::EventMsg(