The yield on short-term Treasurys has been higher than on long-term notes for more than 30 consecutive trading sessions, a sign that investors are concerned about the durability of the decadelong economic expansion. The yield on three-month bills has exceeded that of the benchmark 10-year Treasury note by as much as 0.259 percentage point, the most since May 2007, before the financial crisis. Shorter-term bill yields tend to reflect expectations for Federal Reserve interest-rate policy, while those on longer-term securities…