By Andy Sullivan and David Morgan WASHINGTON (Reuters) -Democratic and Republican leaders in the U.S. Congress on Sunday unveiled a short-term spending bill that would avert a partial government shutdown and keep federal agencies operating into March. The agreement aims to avert short-term chaos and buy more time to craft the complex spending legislation that funds government activity. Government agencies that oversee transportation, housing, and other services are due to run out of funding by midnight on Friday and would have to scale back activity if new funding is not signed into law. The D…