fix(auth): add remove() trap to engine auth storage proxy

The CLI proxy already had a remove() trap, but the engine's
createFusionAuthStorage was missing it. Without this trap, calling
remove() on a provider would delete the credential from storage but
not add it to loggedOutProviders, allowing fallback credentials to
resurrect the provider on the next read.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

packages/engine/src/auth-storage.ts

@@ -183,6 +183,13 @@ export function createFusionAuthStorage(): AuthStorage {
         };
       }
 
+      if (prop === "remove") {
+        return (provider: string) => {
+          target.remove(provider);
+          loggedOutProviders.add(provider);
+        };
+      }
+
       if (prop === "set") {
         return (provider: string, credential: AuthCredential) => {
           target.set(provider, credential);