A delicious rumor is curling through chic Parisian salons like smoke from a perfumed cigarette: The Hotel Lambert, palatial home to Guy de Rothschild and perhaps the most celebrated private residence in the city, is for sale. Some well-informed observers confidently quoted a price of 50 million euros — around $50 million — for the 17th-century house, which was built by François Mansart and redecorated by Mongiardino, and at least one top-drawer realtor has been soliciting potential buyers. But Baron Alexis de Redé, who lives in an apartment there thanks to his friendship with the late Marie-Hélène de Rothschild, denies the rumor. “Completely untruthful,” he sniffs. It all began a couple of months back, reports de Redé, when Guy de Rothschild permitted a film crew to use the front of the house as a set. They hung a huge “For Sale” sign out front as a film prop, and passersby confused life with art.