parseUpgradeJson body failures

Trap: wrapping only fetch() looks sufficient because headers prove a response arrived, but response.json() can still abort or fail while consuming the body. Fix: use parseUpgradeJson() for upgrade metadata; rethrow an aborted signal’s exact reason, classify stream termination as UpgradeTransportError, and classify completed malformed JSON as terminal metadata failure. This distinction keeps automatic cache fallback transport-only and prevents HTTP or malformed-success failures from using stale cached targets.