fn unusable_reason(
usage: Option<&UsageSnapshot>,
last_error: Option<UsageFetchStatus>,
) -> Option<Unusable>Expand description
The classification behind AccountStateMap::unusable_reason for a
key already resolved to its inputs. None means usable: usage
unknown, or under 100% on every window, or the last probe failed
transiently (network / unknown HTTP - we just don’t know yet).
Some means at the cap, or unable to authenticate.
Saturation outranks a failed probe: the windows came from a successful read, and on an at-cap account the failed probe usually IS the cap throttling the endpoint.