Fix unused mut warning in unified persistence fallback test

Author: elnafateh

tests/integration_tests_rust.rs

@@ -3527,7 +3527,7 @@ async fn unified_send_bolt11_persistence_failure_no_onchain_fallback() {
 
 	// Node A (sender) runs on a store that fails writes, so the payment-store insert after
 	// `pay_for_bolt11_invoice` succeeds will surface as `PersistenceFailed`.
-	let mut config_a = random_config();
+	let config_a = random_config();
 	setup_builder!(builder_a, config_a.node_config);
 	let mut sync_config = EsploraSyncConfig::default();
 	sync_config.background_sync_config = None;