chore: move check to internal function

Author: mmsaki

src/router.sol

@@ -42,10 +42,14 @@ contract Router is IRouter {
 
   /// Only allow the PoolManager to call certain functions.
   modifier onlyPoolManager() {
-    require(msg.sender == address(POOLMANAGER));
+    _checkCallerIsPoolManager();
     _;
   }
 
+  function _checkCallerIsPoolManager() internal view {
+    require(msg.sender == address(POOLMANAGER), "Caller is not PoolManager");
+  }
+
   /// @inheritdoc IRouter
   function swap(AsyncOrder calldata order, bytes memory userData) external {
     address onBehalf = address(this);