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/schema/json/v2/ThreadRealtimeItemTranscriptDeltaNotification.json

@@ -0,0 +1,22 @@
+{
+  "$schema": "http://json-schema.org/draft-07/schema#",
+  "description": "EXPERIMENTAL - text appended to an active realtime transcript item.",
+  "properties": {
+    "delta": {
+      "type": "string"
+    },
+    "itemId": {
+      "type": "string"
+    },
+    "threadId": {
+      "type": "string"
+    }
+  },
+  "required": [
+    "delta",
+    "itemId",
+    "threadId"
+  ],
+  "title": "ThreadRealtimeItemTranscriptDeltaNotification",
+  "type": "object"
+}