fix: also update viper default max_line_size from 40MB to 500MB

The viper config default (config.go) was overriding the constant
in gateway_service.go. Both must be updated to take effect.

backend/internal/config/config.go

@@ -1402,7 +1402,7 @@ func setDefaults() {
 	viper.SetDefault("gateway.concurrency_slot_ttl_minutes", 30) // 并发槽位过期时间（支持超长请求）
 	viper.SetDefault("gateway.stream_data_interval_timeout", 180)
 	viper.SetDefault("gateway.stream_keepalive_interval", 10)
-	viper.SetDefault("gateway.max_line_size", 40*1024*1024)
+	viper.SetDefault("gateway.max_line_size", 500*1024*1024)
 	viper.SetDefault("gateway.scheduling.sticky_session_max_waiting", 3)
 	viper.SetDefault("gateway.scheduling.sticky_session_wait_timeout", 120*time.Second)
 	viper.SetDefault("gateway.scheduling.fallback_wait_timeout", 30*time.Second)