Function snapshot_from_openrouter_key 

fn snapshot_from_openrouter_key(
    payload: KeyResponse,
    balance: Option<f64>,
) -> Result<UsageSnapshot, ProbeError>

Map an OpenRouter key payload into a snapshot. Window-free: a pay-per-token key has no plan window and, when uncapped, no denominator, so nothing here synthesises a utilization.

balance is the account-wide figure from the credits fetch, or None when that fetch failed - it never fails the snapshot.

Fallible on purpose. A 200 whose body carries no data envelope, or an envelope with none of the three usage figures, is a response forge cannot read rather than a bill of zero - and since set_usage takes any snapshot to Ready without inspecting it, mapping those to zeroes would report a confident number nothing prompts anyone to doubt. An absent figure alongside a present sibling is a real zero and maps as one.