Expose exact per-response usage in raw app-server events (#32985)

## Why

Clients that account for individual upstream Responses API calls need the exact
usage for each completion rather than accumulated or estimated thread usage.

## What changed

- Add the opt-in `rawResponse/completed` app-server notification with the
  `threadId`, `turnId`, `responseId`, and upstream `usage` payload.
- Emit the transient event for normal turns and compaction requests when
  `experimentalRawEvents` is enabled. The event is not persisted or replayed,
  and `usage` is `null` when the upstream response omits it.
- Export the notification in the app-server schemas and document its behavior.

## Testing

- Cover exact token breakdowns for turn and compaction completions.

GitOrigin-RevId: 49040a060bf1784c1ff1dd62112efb4a778428c4

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

@@ -1645,6 +1645,8 @@ server_notification_definitions! {
     ItemCompleted => "item/completed" (v2::ItemCompletedNotification),
     /// This event is internal-only. Used by Codex Cloud.
     RawResponseItemCompleted => "rawResponseItem/completed" (v2::RawResponseItemCompletedNotification),
+    /// This event is internal-only. Used by clients that need exact upstream usage.
+    RawResponseCompleted => "rawResponse/completed" (v2::RawResponseCompletedNotification),
     AgentMessageDelta => "item/agentMessage/delta" (v2::AgentMessageDeltaNotification),
     /// EXPERIMENTAL - proposed plan streaming deltas for plan items.
     PlanDelta => "item/plan/delta" (v2::PlanDeltaNotification),