php-nextgen - Fix flatten() to support arrays of files

[lukascernydis]

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: string, format: binary).

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

The error is not in generator itself but rather in the generated utility PHP class FormDataProcessor.php

Reproducible by generating method that utilizes any file or file list. For example:

            requestBody:
                required: true
                content:
                    multipart/form-data:
                        schema:
                            type: object
                            required:
                                - files
                            properties:
                                files:
                                    type: array
                                    items:
                                        type: string
                                        format: binary

@renepardon

[wing328]

thanks for the pr

please follow step 3 to update the samples