asim: panic if ticket not found in store rebalancer

This commit adds a panic assertion to ensure pending ticket exists in store
rebalancer asim since asim should not delete a ticket from controller.tickets.

Epic: none
Relese note: none

Author: wenyihu6

pkg/kv/kvserver/asim/storerebalancer/store_rebalancer.go

@@ -145,7 +145,7 @@ func (src *storeRebalancerControl) checkPendingTicket() (done bool, _ error) {
 	ticket := src.rebalancerState.pendingTicket
 	op, ok := src.controller.Check(ticket)
 	if !ok {
-		return true, nil
+		panic(fmt.Sprintf("operation not found for ticket %v", ticket))
 	}
 	done, _ = op.Done()
 	if !done {