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/src/protocol/v2.rs

@@ -3787,6 +3787,17 @@ pub struct ThreadRealtimeItemAddedNotification {
     pub item: JsonValue,
 }
 
+/// EXPERIMENTAL - flat transcript delta emitted whenever realtime
+/// transcript text changes.
+#[derive(Serialize, Deserialize, Debug, Clone, PartialEq, Eq, JsonSchema, TS)]
+#[serde(rename_all = "camelCase")]
+#[ts(export_to = "v2/")]
+pub struct ThreadRealtimeTranscriptUpdatedNotification {
+    pub thread_id: String,
+    pub role: String,
+    pub text: String,
+}
+
 /// EXPERIMENTAL - streamed output audio emitted by thread realtime.
 #[derive(Serialize, Deserialize, Debug, Clone, PartialEq, JsonSchema, TS)]
 #[serde(rename_all = "camelCase")]