NIIGATA, Japan/WASHINGTON, May 10 (Reuters) - Treasury Secretary Janet Yellen lands at Japan's G7 finance meeting this week, with her ambitious economic agenda hijacked by a bitter partisan fight over the U.S. debt ceiling that threatens a new financial crisis, and a signature tax deal blocked by Republicans. Yellen spent last year spearheading punishing G7-led sanctions imposed on Russia over its invasion of Ukraine. She's likely to spend much of the two-day meeting in Niigata reassuring the same group that the U.S. is not going to tank the global economy with a payments default. The shift illustrates how profoundly U.S....