fix: complete first version issue remediation

gateway/internal/queue/messages_test.go

@@ -31,6 +31,7 @@ func TestSubmitCommandUnmarshalsQueuePriority(t *testing.T) {
 		"cmpp": {
 			"serviceId": "SMS",
 			"srcId": "10690000",
+			"extensionDigits": 4,
 			"registeredDelivery": 1,
 			"msgFmt": 8
 		},
@@ -62,4 +63,7 @@ func TestSubmitCommandUnmarshalsQueuePriority(t *testing.T) {
 	if command.Upstream.DesiredConnections != 2 || command.Upstream.WindowSize != 16 {
 		t.Fatalf("unexpected upstream window config: %+v", command.Upstream)
 	}
+	if command.CMPP.ExtensionDigits != 4 {
+		t.Fatalf("unexpected extension digits: %d", command.CMPP.ExtensionDigits)
+	}
 }