Version-check helpers never throw

checkForUpdateInBackgroundImpl() and getUpdateNotificationWithCopy() must never throw, propagate errors, or delay CLI execution. Catch database, network, parsing, cache, eligibility, and notification-construction failures at their boundaries; capture actionable unexpected errors with Sentry.captureException(), but do not report expected aborts, and record transient infrastructure failures on the version-check span when appropriate. Return null when unavailable, rate-limited, current, ineligible, stderr is not a TTY, or any error occurs. Persist last_notified only after successfully producing a notification.