The NVIDIA key-validation chat probe used models[0] (z-ai/glm-5.1), which
requires the 'Public API Endpoints' account permission and has DEGRADED
windows. Accounts lacking that permission see the probe hang until the
validation timeout, surfacing as a misleading 'Upstream Error' on a valid key.
Probe the universally-available meta/llama-3.1-8b-instruct instead, still
overridable via providerSpecificData.validationModelId. + unit test.
