fix(backend): Fix issue with FAPI suffixed cookies disabled (#3789)

Co-authored-by: Nikos Douvlis <nikosdouvlis@gmail.com>
Co-authored-by: Stefanos Anagnostou <anagstef@users.noreply.github.com>

Author: 3 people

.changeset/eleven-suits-flow.md

@@ -0,0 +1,5 @@
+---
+"@clerk/backend": patch
+---
+
+Handle the scenario where FAPI returns unsuffixed cookies without throwing a handshake

packages/backend/src/tokens/authenticateContext.ts

@@ -213,6 +213,14 @@ class AuthenticateContext {
       }
     }
 
+    // If a suffixed session cookie exists but the corresponding client_uat cookie is missing, fallback to using
+    // unsuffixed cookies.
+    // This handles the scenario where an app has been deployed using an SDK version that supports suffixed
+    // cookies, but FAPI for its Clerk instance has the feature disabled (eg: if we need to temporarily disable the feature).
+    if (!suffixedClientUat && suffixedSession) {
+      return false;
+    }
+
     return true;
   }