Replace singular reference_image field with reference_images list to
support an arbitrary number of reference images. Map them to the correct
BFL API parameter names based on model family:
- flux-2-*: input_image, input_image_2, ..., input_image_8
- flux-kontext-*: input_image, input_image_2, ..., input_image_4
- flux 1.x: image_prompt (single)
BREAKING CHANGE: reference_image config field renamed to reference_images (list).
The tests.conftest import could not be resolved in the nix sandbox
because tests is not a proper package. Define the WriteConfig type
alias directly in test_builder.py and test_graph.py instead.
