managed_image_partner inlined the dispatcher's strip-or-None reading of
image_gen.provider/model; managed_route now normalises raw values itself,
so both callers pass config values straight through and cannot drift.
_managed_model_plugin's docstring points at managed_route instead of
restating the rule, and NOUS_MANAGED_PROVIDER is a top-level import (no
cycle). A Krea style reference that disappears between is_file() and open()
now returns invalid_image_url instead of raising. Test comment corrected:
only managed-model routing ignores legacy use_gateway.
62c42e9db9