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/ServerNotification.json

@@ -3068,6 +3068,26 @@
       ],
       "type": "object"
     },
+    "ThreadRealtimeTranscriptUpdatedNotification": {
+      "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"
+      ],
+      "type": "object"
+    },
     "ThreadStartedNotification": {
       "properties": {
         "thread": {
@@ -4565,6 +4585,26 @@
       "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": {