[app-server] feat: add turn/diff/updated event (#7279)

This is the V2 version of `EventMsg::TurnDiff`.

I decided to expose this as a `turn/*` notification as opposed to an
Item to make it more explicit that the diff is accumulated throughout a
turn (every `apply_patch` call updates the running diff). Also, I don't
think it's worth persisting this diff as an Item because it can always
be recomputed from the actual `FileChange` Items.

codex-rs/app-server-protocol/src/protocol/v2.rs

@@ -1141,6 +1141,16 @@ pub struct TurnCompletedNotification {
     pub turn: Turn,
 }
 
+#[derive(Serialize, Deserialize, Debug, Clone, PartialEq, JsonSchema, TS)]
+#[serde(rename_all = "camelCase")]
+#[ts(export_to = "v2/")]
+/// Notification that the turn-level unified diff has changed.
+/// Contains the latest aggregated diff across all file changes in the turn.
+pub struct TurnDiffUpdatedNotification {
+    pub turn_id: String,
+    pub diff: String,
+}
+
 #[derive(Serialize, Deserialize, Debug, Clone, PartialEq, JsonSchema, TS)]
 #[serde(rename_all = "camelCase")]
 #[ts(export_to = "v2/")]