added check for allowing only class and blitz game mode at server

plugins/matchmaking.go

@@ -29,11 +29,12 @@ func MatchmakerMatched(
 
 	propsA := entries[0].GetProperties()
 	propsB := entries[1].GetProperties()
+  validModes := map[string]bool{"classic": true, "blitz": true}
 
 	modeA, okA := propsA["mode"].(string)
 	modeB, okB := propsB["mode"].(string)
 
-	if !okA || !okB {
+  if !okA || !okB || !validModes[modeA] || !validModes[modeB] {
 		logger.Warn("MatchmakerMatched missing mode property — ignoring")
 		return "", nil
 	}