Add experimental Guardian review for injected thread commands (#58119)

## What changed

Expose `thread/guardianReview` to review an idle thread's final injected `exec_command` without executing it. Return the call ID and Guardian assessment, or `null` for the assessment when exec policy skips approval.

Subscribe the requesting connection to the thread, enforce input and model-provider restrictions, and cancel an in-flight review when the connection disconnects so the thread is released promptly.

## Testing

Add integration coverage for assessment responses, policy-skipped reviews, model-provider enforcement, and WebSocket disconnect cancellation.

GitOrigin-RevId: e8ad5598e8f101a937447ebe26136310bb76c8d1

codex-rs/app-server/src/message_processor.rs

@@ -181,6 +181,7 @@ pub(crate) struct MessageProcessor {
 
 #[derive(Debug)]
 pub(crate) struct ConnectionSessionState {
+    disconnect: CancellationToken,
     pub(crate) rpc_gate: Arc<ConnectionRpcGate>,
     pub(crate) origin: crate::transport::ConnectionOrigin,
     pub(crate) mcp_event_streams: McpEventStreams,
@@ -201,6 +202,7 @@ impl ConnectionSessionState {
     pub(crate) fn new(origin: crate::transport::ConnectionOrigin) -> Self {
         Self {
             origin,
+            disconnect: CancellationToken::new(),
             rpc_gate: Arc::new(ConnectionRpcGate::new()),
             mcp_event_streams: McpEventStreams::default(),
             initialized: OnceLock::new(),
@@ -904,6 +906,7 @@ impl MessageProcessor {
         connection_id: ConnectionId,
         session_state: &ConnectionSessionState,
     ) {
+        session_state.disconnect.cancel();
         session_state.rpc_gate.close().await;
         self.account_processor
             .gateway_connection_closed(connection_id);
@@ -1032,6 +1035,7 @@ impl MessageProcessor {
             | ClientRequest::ThreadDelete { .. }
             | ClientRequest::ThreadArchive { .. } => (Some(self.turn_admission.admit()?), false),
             ClientRequest::ThreadSample { .. }
+            | ClientRequest::ThreadGuardianReview { .. }
             | ClientRequest::TurnStart { .. }
             | ClientRequest::TurnSteer { .. }
             | ClientRequest::ReviewStart { .. }
@@ -1704,6 +1708,20 @@ impl MessageProcessor {
                     )
                     .await
             }
+            ClientRequest::ThreadGuardianReview { params, .. } => {
+                let cancellation = &request_context.cancellation;
+                let review =
+                    self.turn_processor
+                        .thread_guardian_review(&request_id, params, cancellation);
+                tokio::pin!(review);
+                tokio::select! {
+                    result = &mut review => result,
+                    _ = session.disconnect.cancelled() => {
+                        cancellation.cancel();
+                        review.await
+                    }
+                }
+            }
             ClientRequest::ThreadInjectItems { params, .. } => {
                 self.turn_processor
                     .thread_inject_items(&request_id, params)