Can an AMI antenna be installed on a streetlight bracket 12″ above the top communication if the bracket is bonded and the electric feed is in conduit or jacketed?

No. NESC Rules 235C and 238 specify the requirements for the communication worker safety zone. Nothing is allowed in this zone except grounded brackets and span wires supporting streetlights and traffic signals, drip loops feeding street lights and traffic signals, and vertical risers in conduit.