[Router] Add the shutdown-drain configuration surface (#39015)

Co-authored-by: Kangyan Zhou <kangyan.zhou@radixark.ai>
Co-authored-by: Claude Opus 5 (1M context) <noreply@anthropic.com>

experimental/sgl-router/tests/proxy/chat_routing.rs

@@ -28,6 +28,7 @@ fn config_for(_worker_url: &str) -> Config {
         server: ServerConfig {
             host: "0".into(),
             port: 0,
+            ..Default::default()
         },
         observability: ObservabilityConfig::default(),
         model: ModelConfig {