The man chosen to run the health insurance giant that would arise out of this week's proposed merger of Tufts Health Plan and Harvard Pilgrim Health Care is not often rattled. With his patrician roots and strong diplomatic skills, James Roosevelt Jr., 65, has frequently been tapped by political and business leaders to deliver an unwelcome message or fix a thorny problem. Now those skills will be put to the test as he spearheads a merger push that is certain to be scrutinized by regulators and consumer advocates. Roosevelt is a health policy insider and behind-the-scenes force in Democratic circles. He is also a lawyer, grandson of Franklin Delano Roosevelt, a guy who prefers diners and neighborhood restaurants over trendy bistros, and a devout Catholic who distributes Communion on Sundays at his church in Cambridge.