Fix KeyError for traffic vehicles missing render_vehicle config

[BridgerB]

Problem

When traffic_density > 0, spawning traffic vehicles fails with:

KeyError: 'render_vehicle'

at metadrive/component/vehicle/base_vehicle.py line 142.

Root Cause

In BaseVehicle.__init__, line 142 accesses the passed vehicle_config parameter directly:

self.render_vehicle = vehicle_config["render_vehicle"]

However, the config merging happens just before this:

1. Line 136: BaseObject.__init__(..., self.engine.global_config["vehicle_config"]) - initializes self._config with the global vehicle_config (which includes render_vehicle=True)
2. Line 138: self.update_config(vehicle_config) - merges the passed config into self._config

For the main agent vehicle, the passed vehicle_config includes render_vehicle, so it works. But for traffic vehicles, the passed config only contains spawn info from traffic_vehicle_config (spawn_lane_index, spawn_longitude, etc.) - it doesn't include render_vehicle.

After the merge, self.config["render_vehicle"] correctly contains True inherited from the base config, but vehicle_config["render_vehicle"] fails because the key was never in the passed parameter.

Fix

Use self.config["render_vehicle"] instead of vehicle_config["render_vehicle"], consistent with how use_special_color is accessed on line 140.