[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/common.rs

@@ -504,6 +504,7 @@ server_notification_definitions! {
     ThreadStarted => "thread/started" (v2::ThreadStartedNotification),
     TurnStarted => "turn/started" (v2::TurnStartedNotification),
     TurnCompleted => "turn/completed" (v2::TurnCompletedNotification),
+    TurnDiffUpdated => "turn/diff/updated" (v2::TurnDiffUpdatedNotification),
     ItemStarted => "item/started" (v2::ItemStartedNotification),
     ItemCompleted => "item/completed" (v2::ItemCompletedNotification),
     AgentMessageDelta => "item/agentMessage/delta" (v2::AgentMessageDeltaNotification),