ASTANA, Kazakhstan — Secretary of State Antony J. Blinken came to Central Asia to press his case that the region should hold the line against Russian efforts to seek economic aid as Moscow grapples with Western sanctions. Within hours of landing in Astana, the snow-draped capital of Kazakhstan, he received a sign that the United States had some leverage. The Kazakh president stood next to Mr. Blinken in the blue-doomed presidential palace and thanked the Americans for their support of his nation’s “independence, territorial integrity and sovereignty.” The president, Kassym-Jomart…