Six months into the new House Democratic majority, long-simmering tensions between the speaker and the squad — Representatives Ocasio-Cortez, Ilhan Omar of Minnesota, Rashida Tlaib of Michigan and Ayanna Pressley of Massachusetts — have boiled over in the most public of ways, setting off a flurry of criticism of Ms. Pelosi among liberal activists and reinvigorating a debate within the party about how best to stand up to President Trump.