ABO SAN LUCAS, Mexico, Oct. 27 — President Bush left a summit conference here today without a pledge from Mexico to support the American resolution in the United Nations Security Council to disarm Iraq. Mexican officials made it clear that Mexico is siding with France in the debate at the United Nations. While the United States is demanding one resolution that includes a legal basis for the military action against Iraq, France wants two stages, authorizing force only when Baghdad fails to comply with weapons inspections.