fix: 修复渠道一致性问题

model/cache.go

@@ -258,8 +258,8 @@ func CacheGetRandomSatisfiedChannel(group string, model string) (*Channel, error
 			return channel, nil
 		}
 	}
-	// return the last channel if no channel is found
-	return channels[endIdx-1], nil
+	// return null if no channel is not found
+	return nil, errors.New("channel not found")
 }
 
 func CacheGetChannel(id int) (*Channel, error) {