Add isolated reviews to the guardian reviewer pool (#57522)

## What changed

Add `ReviewerPool::review_isolated` to run a review in a temporary session without reading or changing the reusable reviewer. Build the request context without a previous session and spawn without a cached snapshot.

## Testing

Add a test verifying that an isolated review receives no snapshot, uses a separate session, and preserves the prewarmed session for the next regular review.

GitOrigin-RevId: 6cceff0c399f30f3c3303b0bf5232c274c902e3a

codex-rs/ext/guardian-reviewer/src/pool.rs

@@ -329,6 +329,19 @@ impl<S: ReviewerSession> ReviewerPool<S> {
         (outcome, analytics)
     }
 
+    /// Reviews in a temporary session without reading or changing the reusable reviewer.
+    /// The request builds its context without a previous session or cached snapshot.
+    pub async fn review_isolated<R>(
+        &self,
+        request: &R,
+    ) -> (GuardianReviewSessionOutcome, GuardianReviewAnalyticsResult)
+    where
+        R: ReviewerRequest<Session = S>,
+    {
+        let context = request.context(/*previous*/ None);
+        Box::pin(self.review_ephemeral(request, context, /*snapshot*/ None)).await
+    }
+
     async fn review_ephemeral<R>(
         &self,
         request: &R,

codex-rs/ext/guardian-reviewer/src/pool_tests.rs

@@ -188,3 +188,38 @@ async fn fresh_attempt_bypasses_cached_session_and_snapshot(busy: bool) {
     .await;
     assert_eq!(selections.recv().await, Some(if busy { 0 } else { 1 }));
 }
+
+#[tokio::test]
+async fn isolated_review_preserves_reusable_session() {
+    let spawns = Arc::new(AtomicUsize::default());
+    let pool = ReviewerPool::new(
+        Arc::new(ReviewerTasks::default()),
+        move |_, _, _, snapshot, _| {
+            assert_eq!(
+                snapshot, None,
+                "isolated review must not inherit a snapshot"
+            );
+            let id = spawns.fetch_add(/*val*/ 1, Ordering::SeqCst);
+            Box::pin(async move { Ok(Session(id)) })
+        },
+    );
+    pool.prewarm(Arc::new(()), ()).await.expect("prewarm trunk");
+    let (selected, mut selections) = tokio::sync::mpsc::unbounded_channel();
+
+    pool.review_isolated(&Request {
+        fresh: false,
+        selected: selected.clone(),
+        release: None,
+    })
+    .await;
+    assert_eq!(selections.recv().await, Some(1));
+
+    pool.review(Request {
+        fresh: false,
+        selected,
+        release: None,
+    })
+    .await;
+    assert_eq!(selections.recv().await, Some(0));
+    pool.shutdown().await;
+}