Function is_subscription_usage_error 

pub fn is_subscription_usage_error(err: &str) -> bool

Returns true if err looks like a failure surfaced by the Anthropic subscription-usage poll path (/v1/messages/usage / /api/oauth/usage). Used by clients as defence-in-depth: the server-side handler in #940 no longer sends Response::Error for these failures — it falls back to a cached or default Response::SubscriptionUsage — but if a future code path were to regress and emit such an error over the wire, clients can recognize it as out-of-band and refrain from tearing down streaming UI state (in-flight tool calls, agent phase, etc.).