fix: add reference_image to FILE_FIELDS for proper file handling

[AdirAmsalem]

Summary

Fixes a bug where reference_image was missing from FILE_FIELDS in both the sync and queue clients. This caused reference_image to bypass the file-field handling path, meaning passing a file path or file-like object as reference_image would fail at validation — only raw bytes worked by accident.

Flagged by Cursor Bugbot on #43.

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Note

Low Risk
Low risk: a small input-handling change that only affects how reference_image is routed through validation/file processing in the sync and queue clients.

Overview
Fixes reference_image handling in both sync process() and async queue submissions by adding it to the FILE_FIELDS set.

This ensures reference_image goes through the same file-input path (placeholder-bytes validation + final override) as data/start/end, so file paths or file-like objects are accepted consistently.

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