As Mayor Giuliani looks over the acres of gleaming tractors, mammoth cotton harvesters, and hightech cow milking equipment at a fair in California's vast agricultural heartland tomorrow morning, he will surely feel like he's a long way from Manhattan. The prospect of a visit from the man billed as America's mayor already has local folks in California's central valley atwitter. "Honestly, this is probably the biggest name we've had," the chairman of the World Ag Expo in Tulare, Calif., Bruce Shannon, said yesterday. Mr. Shannon allowed that Mr. Giuliani had "probably not run into a lot of combines and tractors"...