[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/common/mod.rs

@@ -6,3 +6,11 @@
 pub mod cache_aware_fixture;
 pub mod mock_worker;
 pub mod streaming;
+
+#[allow(dead_code)] // not every test file inspects forwarded rids
+pub fn is_engine_shaped_rid(rid: &str) -> bool {
+    rid.len() == 32
+        && rid
+            .bytes()
+            .all(|b| b.is_ascii_hexdigit() && !b.is_ascii_uppercase())
+}