Combine first 6 ingredients in a food processor; process until smooth. Cover and chill. Combine tomato, cucumber, onion, and dressing in a bowl; cover and marinate in refrigerator 1 hour. Drain tomato mixture; discard marinade. Line each pita half with a lettuce leaf; spread each with 3 tablespoons chickpea mixture. Spoon about 2/3 cup marinated vegetables into each pita half.