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.
Implement bulkgen/providers/bfl.py with a fully async httpx-based client
that supports all current and future BFL models (including flux-2-*).
Remove the blackforest dependency and simplify the image provider by
eliminating the asyncio.to_thread wrapper.