Add typehints to flatten exception.

This makes the PHPStan baseline empty 🎉.

Author: nickygerritsen

phpstan-baseline.neon

@@ -22,5 +22,4 @@ parameters:
             message: "#Method .* return type has no value type specified in iterable type array#"
             path: webapp/src/DataFixtures/Test
 includes:
-    - phpstan-baseline.neon
     - lib/vendor/phpstan/phpstan-doctrine/extension.neon

phpstan.dist.neon

@@ -38,6 +38,11 @@ public static function getSubscribingMethods(): array
         ];
     }
 
+    /**
+     * @param array{params: string[]} $type
+     *
+     * @return array{code: int, message: string}
+     */
     public function serializeToJson(
         JsonSerializationVisitor $visitor,
         FlattenException $exception,