sgl-router: experimental Rust HTTP router for SGLang worker pools (#25851)

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

experimental/sgl-router/tests/component/policies/round_robin.rs

@@ -0,0 +1,56 @@
+// SPDX-FileCopyrightText: Copyright (c) 2026 The SGLang Authors
+// SPDX-License-Identifier: Apache-2.0
+
+use sgl_router::discovery::{ModelId, WorkerId, WorkerMode, WorkerSpec};
+use sgl_router::policies::round_robin::RoundRobinPolicy;
+use sgl_router::policies::{Policy, SelectionContext};
+use sgl_router::workers::Worker;
+use std::sync::Arc;
+
+fn worker(id: &str) -> Arc<Worker> {
+    Arc::new(Worker::new(WorkerSpec {
+        id: WorkerId(id.into()),
+        url: format!("http://{id}"),
+        mode: WorkerMode::Plain,
+        model_ids: vec![ModelId("m".into())],
+        bootstrap_port: None,
+    }))
+}
+
+#[test]
+fn cycles_through_workers() {
+    let p = RoundRobinPolicy::new();
+    let ws = vec![worker("a"), worker("b"), worker("c")];
+    let model_id = ModelId("m".into());
+    let ctx = SelectionContext::new(&model_id, None);
+    let picks: Vec<_> = (0..6)
+        .filter_map(|_| p.select(&ws, &ctx))
+        .map(|w| w.id.0.clone())
+        .collect();
+    assert_eq!(picks, vec!["a", "b", "c", "a", "b", "c"]);
+}
+
+#[test]
+fn empty_pool_returns_none() {
+    let p = RoundRobinPolicy::new();
+    let ws: Vec<Arc<Worker>> = vec![];
+    let model_id = ModelId("m".into());
+    let ctx = SelectionContext::new(&model_id, None);
+    assert!(p.select(&ws, &ctx).is_none());
+}
+
+#[test]
+fn distribution_across_100_calls() {
+    let p = RoundRobinPolicy::new();
+    let ws = vec![worker("a"), worker("b"), worker("c")];
+    let model_id = ModelId("m".into());
+    let ctx = SelectionContext::new(&model_id, None);
+    let mut counts = std::collections::HashMap::new();
+    for _ in 0..99 {
+        let w = p.select(&ws, &ctx).unwrap();
+        *counts.entry(w.id.0.clone()).or_insert(0) += 1;
+    }
+    assert_eq!(counts["a"], 33);
+    assert_eq!(counts["b"], 33);
+    assert_eq!(counts["c"], 33);
+}