[Router] Add load-aware prefill admission and bounded policy proposals (#37843)

Signed-off-by: Vincent Gao <vincentbo@linux.alibaba.com>
Co-authored-by: Kangyan Zhou <zky314343421@gmail.com>
Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Co-authored-by: Shangming Cai <csmthu@gmail.com>

experimental/sgl-router/tests/e2e/infra/test_model_pool.py

@@ -0,0 +1,50 @@
+import signal
+
+from infra import model_pool
+
+
+class _Process:
+    pid = 1234
+
+    def __init__(self):
+        self.wait_timeouts = []
+
+    def poll(self):
+        return None
+
+    def send_signal(self, sig):
+        raise AssertionError(f"signaled only the parent process: {sig}")
+
+    def wait(self, timeout=None):
+        self.wait_timeouts.append(timeout)
+        return 0
+
+
+def test_shutdown_waits_for_the_worker_process_group(monkeypatch):
+    process = _Process()
+    signals = []
+    probes = iter([True, True, False])
+
+    def killpg(pgid, sig):
+        signals.append((pgid, sig))
+        if sig == 0 and not next(probes):
+            raise ProcessLookupError
+
+    monkeypatch.setattr(model_pool.os, "killpg", killpg)
+    monkeypatch.setattr(model_pool.time, "sleep", lambda _: None)
+
+    instance = model_pool.ModelInstance(
+        url="http://127.0.0.1:30000",
+        port=30000,
+        process=process,
+        model_id="qwen3-0.6b",
+    )
+    instance.shutdown()
+
+    assert signals == [
+        (process.pid, signal.SIGTERM),
+        (process.pid, 0),
+        (process.pid, 0),
+        (process.pid, 0),
+    ]
+    assert process.wait_timeouts == [60]