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/codex_app_server_protocol.v2.schemas.json

@@ -8709,6 +8709,26 @@
           "title": "Thread/realtime/itemAddedNotification",
           "type": "object"
         },
+        {
+          "properties": {
+            "method": {
+              "enum": [
+                "thread/realtime/transcriptUpdated"
+              ],
+              "title": "Thread/realtime/transcriptUpdatedNotificationMethod",
+              "type": "string"
+            },
+            "params": {
+              "$ref": "#/definitions/ThreadRealtimeTranscriptUpdatedNotification"
+            }
+          },
+          "required": [
+            "method",
+            "params"
+          ],
+          "title": "Thread/realtime/transcriptUpdatedNotification",
+          "type": "object"
+        },
         {
           "properties": {
             "method": {
@@ -10802,6 +10822,28 @@
       "title": "ThreadRealtimeStartedNotification",
       "type": "object"
     },
+    "ThreadRealtimeTranscriptUpdatedNotification": {
+      "$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"
+    },
     "ThreadResumeParams": {
       "$schema": "http://json-schema.org/draft-07/schema#",
       "description": "There are three ways to resume a thread: 1. By thread_id: load the thread from disk by thread_id and resume it. 2. By history: instantiate the thread from memory and resume it. 3. By path: load the thread from disk by path and resume it.\n\nThe precedence is: history > path > thread_id. If using history or path, the thread_id param will be ignored.\n\nPrefer using thread_id whenever possible.",