Throttle opcodes: scheduler emits only THROTTLE and RESUME

In switch_scheduler.sv only THROTTLE (4'h9) and RESUME (4'hb) are generated, and only in response to request/grant conditions. STOP (4'ha) and MAX_BW (4'hf) have no scheduler-side source and must be injected externally, e.g., via a testbench response override on the resp path. Don't search the scheduler for their generation logic.