php-nextgen - Fix flatten() to support arrays of files in multipart/form-data

Previously, FormDataProcessor::flatten() unconditionally passed all values through ObjectSerializer::toString(),
which caused an error when flattening arrays containing file resources (e.g. for OpenAPI
multipart/form-data definitions with `type: array`, `items: type: string, format: binary`).

This change adds a check for is_resource() to preserve stream handles without serialization,
ensuring correct behavior when uploading multiple files in a single request.

Author: lukascernydis

modules/openapi-generator/src/main/resources/php-nextgen/FormDataProcessor.mustache

@@ -104,7 +104,11 @@ class FormDataProcessor
                     $currentName .= $currentSuffix;
                 }
 
-                $result[$currentName] = ObjectSerializer::toString($val);
+                if (is_resource($val)) {
+                    $result[$currentName] = $val;
+                } else {
+                    $result[$currentName] = ObjectSerializer::toString($val);
+                }
             }
 
             $currentName = $start;