Multipart and form-data in API steps

API steps now handle the request bodies real APIs actually expect.

• Multipart/form-data body type: select multipart/form-data from the body type dropdown in any API step.
• File uploads in API requests: attach files as part of the request body for endpoints that expect file uploads.
• Mixed payloads: combine file fields and form fields in the same request, with no manual encoding.

If you've been blocked on testing file upload endpoints, document submission flows, or anything that expects multipart payloads, the API step handles it natively now.

#Sprint27