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-protocol/src/protocol/thread_history.rs

@@ -401,6 +401,7 @@ impl ThreadHistoryBuilder {
             | RolloutItem::InterAgentCommunicationMetadata { .. }
             | RolloutItem::TurnContext(_)
             | RolloutItem::WorldState(_)
+            | RolloutItem::RealtimeItem(_)
             | RolloutItem::SecurityRiskScore(_)
             | RolloutItem::SessionMeta(_) => {}
         }