Everywhere else in coderai an engine is what RUNS a model. `ocr.default_engine`
borrows the word for two different kinds of thing: paddle and docTR are runners
with their own weights, while surya-2 and olmOCR are pipelines over a model that
vLLM runs. Only the second kind has to be told where a server is — and that is
the same question the model layer already answers for everything else.

So answer it in all the places a model can actually be served. A cluster node
was the one that was missing: a node is a whole coderai registered with the head
as a remote ENGINE, so nothing in the backend layer knows it exists, and a VLM
hosted there came back as "not served over HTTP" from an install that was
serving it perfectly well. The node's own catalogue is the authority, so it is
asked. An explicit backend pin still wins — discovery is the fallback, not an
override — and the nodes are not probed at all when clustering is off, which
would have added a timeout to the first page of every document.

Also: one test here was passing for the wrong reason. `from codai.models import
manager` reads the package attribute and never consults sys.modules, so a fake
installed there was simply not seen. Patched as attributes of the real module,
which is what made the precedence test fail honestly.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01PxYHWbCAjjjD7A1rhFKtaW