fix: send multipart encoding when using gcs_uri (fixes 400 error) (#278)

FastAPI requires multipart/form-data for endpoints with File()/Form()
parameters. When gcs_uri is used without a file upload, requests
defaulted to x-www-form-urlencoded, causing a 400 Bad Request.

Fix: send a dummy empty file field to force multipart encoding.

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>