Slain cop Jonathan Diller’s brother in blue broke down in a Queens courtroom Thursday as he delivered heart-wrenching testimony against the career criminal who allegedly gunned down the NYPD hero right before his eyes. “I saw Diller laying on the floor … He’s not responding,” Officer Veckash Khedna testified, tears streaming down his face the moment he took the witness stand at accused cop killer Guy Rivera’s murder trial. “I’m thinking he’s dead. I started crying. One of my best friends is