By Gram Slattery, James Oliphant and Nathan Layne SEABROOK, New Hampshire (Reuters) -Florida Governor Ron DeSantis ended his fading presidential campaign on Sunday and endorsed Donald Trump just two days before the pivotal New Hampshire primary, leaving former U.N. Ambassador Nikki Haley as Trump’s last long-shot challenger for the Republican nomination. DeSantis’ decision, less than a week after his deflating loss to Trump in Iowa despite an enormous investment there, caps a stunning fall from grace after DeSantis had been widely seen as Republicans’ most promising alternative to Trump ahead …