By Valentyn Ogirenko and Aziz El Yaakoubi KYIV/RIYADH (Reuters) -Russia and Ukraine carried out an unexpected prisoner swap on Wednesday, the largest since the war began and involving almost 300 people, including 10 foreigners and the commanders who led a prolonged Ukrainian defence of Mariupol earlier this year. The foreigners released included two Britons and a Moroccan who had been sentenced to death in June after being captured fighting for Ukraine. Also freed were three other Britons, two Americans, a Croatian, and a Swedish national. The timing and magnitude of the swap came as a surpris…