BRUSSELS — Ursula von der Leyen has summoned her team of European commissioners to a meeting to try to defuse mounting tensions and improve the way they work. The meeting is set for Feb. 4 in Leuven and is open to all members of the College, though attendance is not mandatory, according to a Commission official involved in organizing the event. The idea for such a meeting was conceived after tense exchanges between commissioners and frustration at the repeated late arrival of files on the desks