Add realtime transcript notification in v2

Emit a typed realtime handoff transcript notification and wire the app-server TUI to handle it.

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

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

@@ -0,0 +1,46 @@
+{
+  "$schema": "http://json-schema.org/draft-07/schema#",
+  "definitions": {
+    "ThreadRealtimeTranscriptEntry": {
+      "properties": {
+        "role": {
+          "type": "string"
+        },
+        "text": {
+          "type": "string"
+        }
+      },
+      "required": [
+        "role",
+        "text"
+      ],
+      "type": "object"
+    }
+  },
+  "description": "EXPERIMENTAL - flat transcript snapshot emitted when realtime hands off to the main agent.",
+  "properties": {
+    "handoffId": {
+      "type": "string"
+    },
+    "itemId": {
+      "type": "string"
+    },
+    "threadId": {
+      "type": "string"
+    },
+    "transcript": {
+      "items": {
+        "$ref": "#/definitions/ThreadRealtimeTranscriptEntry"
+      },
+      "type": "array"
+    }
+  },
+  "required": [
+    "handoffId",
+    "itemId",
+    "threadId",
+    "transcript"
+  ],
+  "title": "ThreadRealtimeTranscriptAddedNotification",
+  "type": "object"
+}