[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/component/discovery/static_urls.rs

@@ -124,6 +124,7 @@ async fn static_urls_pd_role_resolved_end_to_end() {
         server: ServerConfig {
             host: "127.0.0.1".into(),
             port: 0,
+            ..Default::default()
         },
         observability: ObservabilityConfig::default(),
         model: sgl_router::config::ModelConfig {