[router] Improve the router e2e tests (#10102)

sgl-router/py_test/integration/test_payload_size.py

@@ -0,0 +1,33 @@
+import pytest
+import requests
+
+
+@pytest.mark.integration
+def test_payload_size_limit(router_manager, mock_workers):
+    # Start one backend and a router with a 1MB payload limit
+    _, urls, _ = mock_workers(n=1)
+    rh = router_manager.start_router(
+        worker_urls=urls,
+        policy="round_robin",
+        extra={"max_payload_size": 1 * 1024 * 1024},  # 1MB
+    )
+
+    # Payload just under 1MB should succeed
+    payload_small = {
+        "model": "test-model",
+        "prompt": "x" * int(0.5 * 1024 * 1024),  # ~0.5MB
+        "max_tokens": 1,
+        "stream": False,
+    }
+    r = requests.post(f"{rh.url}/v1/completions", json=payload_small)
+    assert r.status_code == 200
+
+    # Payload over 1MB should fail with 413
+    payload_large = {
+        "model": "test-model",
+        "prompt": "x" * int(1.2 * 1024 * 1024),  # ~1.2MB
+        "max_tokens": 1,
+        "stream": False,
+    }
+    r = requests.post(f"{rh.url}/v1/completions", json=payload_large)
+    assert r.status_code == 413