[sgl-router] refactor - move policy-required states under src/state (#40272)

Co-authored-by: Claude Fable 5.1 <noreply@anthropic.com>

experimental/sgl-router/tests/component/workers/manager.rs

@@ -740,7 +740,7 @@ async fn removed_awaits_pending_added() {
 /// independently — 2N round-trips for N workers.
 #[tokio::test]
 async fn manager_emits_single_server_info_fetch_per_worker() {
-    use sgl_router::policies::kv_events::KvEventIndex;
+    use sgl_router::state::kv_events::KvEventIndex;
 
     let body = json!({
         "served_model_name": "m",