Use strong arg type instead of assert

src/Builder/Encoder/OperatorEncoder.php

@@ -101,10 +101,8 @@ private function encodeAsDollarObject(OperatorInterface $value): stdClass
     /**
      * $group stage have a specific encoding because the _id argument is required and others are variadic
      */
-    private function encodeAsGroup(OperatorInterface $value): stdClass
+    private function encodeAsGroup(GroupStage $value): stdClass
     {
-        assert($value instanceof GroupStage);
-
         $result = new stdClass();
         $result->_id = $this->recursiveEncode($value->_id);
 
@@ -132,10 +130,8 @@ private function encodeAsObject(OperatorInterface $value): stdClass
             : $this->wrap($value, $result);
     }
 
-    private function encodeAsSearch(OperatorInterface $value): stdClass
+    private function encodeAsSearch(SearchStage|SearchMetaStage $value): stdClass
     {
-        assert($value instanceof SearchStage || $value instanceof SearchMetaStage);
-
         $result = new stdClass();
         foreach (get_object_vars($value) as $key => $val) {
             // Skip optional arguments. If they have a default value, it is resolved by the server.
@@ -147,7 +143,7 @@ private function encodeAsSearch(OperatorInterface $value): stdClass
             if ($key === 'operator') {
                 $val = $this->recursiveEncode($val);
                 assert($val instanceof stdClass);
-                foreach (get_object_vars($val) as $subkey => $subval) {
+                foreach ($val as $subkey => $subval) {
                     $result->{$subkey} = $subval;
                 }