Fix multiple image edit uploads

[jbeckwith-oai]

Summary

• encode multiple image edit uploads using the API's bracketed image[] multipart field
• preserve the existing scalar image field for single-image edits
• cover both non-streaming and streaming image edits with multipart regression tests

Root cause

The generated multipart encoder flattened arrays of file inputs but reused the scalar field name for every part. The Images API therefore received duplicate image parameters instead of an image[] array and returned duplicate_parameter.

This applies the array field name only to image edit requests, avoiding a behavior change for unrelated multipart array parameters.

Fixes #201.

Validation

• ruby -Itest test/openai/resources/images_test.rb --name '/test_edit_(encodes_multiple_images_as_an_array_multipart_field|preserves_the_scalar_image_multipart_field)/'
• ruby -Itest test/openai/internal/util_test.rb
• rubocop --force-exclusion lib/openai/resources/images.rb test/openai/resources/images_test.rb
• live API contract check: the request now reaches image validation (invalid_image_file) instead of failing with duplicate_parameter