[Router] Abort the engine when a client disconnects mid-request (#39461)

Co-authored-by: Kangyan Zhou <kangyan.zhou@radixark.ai>
Co-authored-by: Claude Opus 5 (1M context) <noreply@anthropic.com>
Co-authored-by: Shangming Cai <csmthu@gmail.com>
Co-authored-by: Kan Wu <wukanustc@gmail.com>

experimental/sgl-router/tests/proxy/h2c_forward.rs

@@ -72,6 +72,7 @@ async fn h2c_client_reaches_http2_only_worker() {
             "/v1/chat/completions",
             &axum::http::HeaderMap::new(),
             Bytes::from_static(b"{}"),
+            None,
         )
         .await
         .expect("h2c client must reach an HTTP/2-only worker");
@@ -96,6 +97,7 @@ async fn http1_client_cannot_reach_http2_only_worker() {
             "/v1/chat/completions",
             &axum::http::HeaderMap::new(),
             Bytes::from_static(b"{}"),
+            None,
         )
         .await;
     assert!(
@@ -164,6 +166,7 @@ async fn h2c_client_streams_sse_from_http2_only_worker() {
             &axum::http::HeaderMap::new(),
             Bytes::from_static(b"{}"),
             None,
+            None,
             Some(Box::new(move || {
                 flag.store(true, std::sync::atomic::Ordering::SeqCst);
             })),