[router] Improve SGLang chat render parity (#39133)

Co-authored-by: Claude Fable 5.1 <noreply@anthropic.com>
Co-authored-by: Shangming Cai <csmthu@gmail.com>

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

@@ -168,3 +168,63 @@ async fn multimodal_request_omits_input_ids() {
         "multimodal requests must not forward input_ids; got {body}"
     );
 }
+
+/// Caller-supplied `input_ids` are never re-rendered or replaced: a flat u32
+/// array (empty included) drives routing, anything else yields no routing
+/// tokens, and the body reaches the engine byte-for-byte for validation.
+#[tokio::test]
+async fn caller_input_ids_are_used_for_routing_and_preserved() {
+    let mock = MockWorker::start(vec![]).await;
+    let ctx = build_ctx(mock.url.clone());
+    for (ids, expected) in [
+        (json!([7, 8]), Some(vec![7, 8])),
+        (json!([]), Some(vec![])),
+        (json!([7, -1]), None),
+        (json!("bad"), None),
+    ] {
+        let request = json!({
+            "model": MODEL,
+            "messages": [{"role": "user", "content": "hi"}],
+            "input_ids": ids,
+        });
+        let tokens = sgl_router::policies::request_tokens_for(
+            &ctx.tokenizers,
+            &ModelId(MODEL.into()),
+            &request,
+        );
+        assert!(!tokens.as_ref().is_some_and(|t| t.rendered_from_chat));
+        assert_eq!(tokens.map(|t| t.ids), expected, "input_ids: {ids}");
+        assert_eq!(
+            send(Arc::clone(&ctx), request.clone()).await,
+            StatusCode::OK
+        );
+        assert_eq!(captured(&mock), request, "body must be forwarded untouched");
+    }
+    // Bypasses are not rendering failures.
+    assert!(!ctx
+        .metrics
+        .render()
+        .contains("sgl_router_ingress_tokenize_errors_total{"));
+}
+
+/// `input_ids: null` is the same as absent: the router renders and forwards.
+#[tokio::test]
+async fn null_input_ids_keep_normal_rendering() {
+    let mock = MockWorker::start(vec![]).await;
+    let ctx = build_ctx(mock.url.clone());
+    let status = send(
+        ctx,
+        json!({
+            "model": MODEL,
+            "messages": [{"role": "user", "content": "hello there friend"}],
+            "input_ids": null,
+        }),
+    )
+    .await;
+    assert_eq!(status, StatusCode::OK);
+    let body = captured(&mock);
+    assert!(
+        body["input_ids"].as_array().is_some_and(|a| !a.is_empty()),
+        "null input_ids must not suppress rendering; got {body}"
+    );
+}