By ELISABETH BUMILLER WASHINGTON, July 14 - In the annals of Washington conspiracy theories, the latest one, about Vice President Dick Cheney's future on the Republican ticket, is as ingenious as it is far-fetched. But that has not stopped it from racing through Republican and Democratic circles like the latest low-carb diet. The newest theory - advanced privately by prominent Democrats, including members of Congress - holds that Mr. Cheney recently dismissed his personal doctor so that he could see a new one, who will conveniently tell him in August that his heart problems make him unfit to run with...