NEW YORK—Wall Street banks have been among the biggest beneficiaries of the pandemic-era trading boom, fueled by the Federal Reserve’s massive injection of cash into financial markets. With the central bank nearing the time when it will start winding down its asset purchases, banks are set to profit again as increased volatility encourages clients to buy and sell more stocks and bonds, analysts, investors, and executives say. The Fed has been buying up government-backed bonds since March 2020, adding $4 trillion to its balance sheet, as part of an emergency response to the COVID-19 pandemic. The strategy was designed to stabilize financial markets and ensure companies and other borrowers had sufficient access to capital. It succeeded but also resulted in unprecedented levels of liquidity, helping equity and bond traders enjoy their most profitable period since the 2007-09 financial crisis. The top five Wall Street investment banks—JP Morgan Chase & Co, …