cmd/devp2p: close the iterator in discv4 LookupRandom (#35206)

The LookupRandom RPC handler obtains a node iterator from
RandomNodes() but never closes it.

RandomNodes() returns a lookupIterator backed by a cancelable
context derived from the listener's lifetime context
(newLookupIterator -> context.WithCancel). The iterator's
Close() is what calls the cancel func, so each LookupRandom call
that never closes leaks the cancel func (and any in-flight lookup
goroutine) until the discv4 listener shuts down. When the listener
serves the RPC API (discv4 --rpc) it is long-lived, so repeated
LookupRandom calls accumulate the leak.

Close the iterator when the handler returns. This matches how the
crawler already releases the same RandomNodes() iterators
(crawl.go closes every iterator it consumes).

cmd/devp2p/discv4cmd.go

@@ -430,6 +430,7 @@ type discv4API struct {
 
 func (api *discv4API) LookupRandom(n int) (ns []*enode.Node) {
 	it := api.host.RandomNodes()
+	defer it.Close()
 	for len(ns) < n && it.Next() {
 		ns = append(ns, it.Node())
 	}