[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/protocol/src/protocol.rs

@@ -1221,6 +1221,9 @@ pub enum EventMsg {
     /// Backend recommends additional account verification for this turn.
     ModelVerification(ModelVerificationEvent),
 
+    /// Backend moderation metadata intended for first-party turn presentation.
+    TurnModerationMetadata(TurnModerationMetadataEvent),
+
     /// Conversation history was compacted (either automatically or manually).
     ContextCompacted(ContextCompactedEvent),
 
@@ -1885,6 +1888,11 @@ pub struct ModelVerificationEvent {
     pub verifications: Vec<ModelVerification>,
 }
 
+#[derive(Debug, Clone, Deserialize, Serialize, PartialEq, JsonSchema, TS)]
+pub struct TurnModerationMetadataEvent {
+    pub metadata: Value,
+}
+
 #[derive(Debug, Clone, Deserialize, Serialize, JsonSchema, TS)]
 pub struct ContextCompactedEvent;