chore: refresh onnx-weekly lock pin (expired nightly wheel) (#291)

The lockfile pinned onnx-weekly 1.20.0.dev20250901, a nightly build
that has since been garbage-collected from the package index, so
'poetry install' failed on every CI job before any test ran. Refresh
the pin to a currently-available nightly (1.23.0.dev20260706) to
unblock CI repo-wide. Lockfile only; pyproject unchanged.