[codex] Forward turn moderation metadata through app-server (#25710)

## Why
First-party backends can supply turn-scoped moderation metadata that
app-server clients need for client-side presentation. Exposing this as
an experimental typed notification lets opted-in clients consume it
without interpreting raw Responses API events.

## What changed
- forward `response.metadata.openai_chatgpt_moderation_metadata` from
Responses API SSE and WebSocket streams as turn-scoped moderation
metadata
- emit the experimental app-server v2 `turn/moderationMetadata`
notification with `{ threadId, turnId, metadata }`
- add app-server integration coverage for the typed moderation metadata
notification

## Testing
- `just test -p codex-core
build_ws_client_metadata_includes_window_lineage_and_turn_metadata`
- `just test -p codex-core` (fails locally: 46 failures and 1 timeout,
primarily missing `test_stdio_server` and shell snapshot timeouts)
- `just test -p codex-app-server-protocol`
- `just test -p codex-app-server
turn_moderation_metadata_emits_typed_notification_v2`
- `just test -p codex-app-server` (fails locally: 792 passed, 10 failed,
and 5 timed out; failures are in existing environment-sensitive tests,
primarily because nested macOS `sandbox-exec` is not permitted)
- `just write-app-server-schema --experimental --schema-root
/tmp/codex-app-server-schema-experimental`

codex-rs/codex-api/src/endpoint/responses_websocket.rs

@@ -683,6 +683,7 @@ async fn run_websocket_response_stream(
                     }
                 };
                 let model_verifications = event.model_verifications();
+                let turn_moderation_metadata = event.turn_moderation_metadata();
                 if event.kind() == "codex.rate_limits" {
                     if let Some(snapshot) = parse_rate_limit_event(&text) {
                         let _ = tx_event.send(Ok(ResponseEvent::RateLimits(snapshot))).await;
@@ -707,6 +708,16 @@ async fn run_websocket_response_stream(
                         "response event consumer dropped".to_string(),
                     ));
                 }
+                if let Some(metadata) = turn_moderation_metadata
+                    && tx_event
+                        .send(Ok(ResponseEvent::TurnModerationMetadata(metadata)))
+                        .await
+                        .is_err()
+                {
+                    return Err(ApiError::Stream(
+                        "response event consumer dropped".to_string(),
+                    ));
+                }
                 match process_responses_event(event) {
                     Ok(Some(event)) => {
                         let is_completed = matches!(event, ResponseEvent::Completed { .. });