Add realtime transcript notification in v2 (#15344)

- emit a typed `thread/realtime/transcriptUpdated` notification from
live realtime transcript deltas
- expose that notification as flat `threadId`, `role`, and `text` fields
instead of a nested transcript array
- continue forwarding raw `handoff_request` items on
`thread/realtime/itemAdded`, including the accumulated
`active_transcript`
- update app-server docs, tests, and generated protocol schema artifacts
to match the delta-based payloads

---------

Co-authored-by: Codex <noreply@openai.com>

codex-rs/app-server-protocol/schema/json/v2/ThreadRealtimeTranscriptUpdatedNotification.json

@@ -0,0 +1,22 @@
+{
+  "$schema": "http://json-schema.org/draft-07/schema#",
+  "description": "EXPERIMENTAL - flat transcript delta emitted whenever realtime transcript text changes.",
+  "properties": {
+    "role": {
+      "type": "string"
+    },
+    "text": {
+      "type": "string"
+    },
+    "threadId": {
+      "type": "string"
+    }
+  },
+  "required": [
+    "role",
+    "text",
+    "threadId"
+  ],
+  "title": "ThreadRealtimeTranscriptUpdatedNotification",
+  "type": "object"
+}