[router] Introduce router integration tests (#10086)

sgl-router/py_test/integration/test_fault_tolerance.py

@@ -0,0 +1,36 @@
+import concurrent.futures
+import subprocess
+import time
+
+import pytest
+import requests
+
+
+@pytest.mark.integration
+def test_worker_crash_reroute_with_retries(router_manager, mock_workers):
+    # Start one healthy and one that will crash on first request
+    _, [ok_url], _ = mock_workers(n=1)
+    _, [crash_url], _ = mock_workers(n=1, args=["--crash-on-request"])
+    rh = router_manager.start_router(
+        worker_urls=[crash_url, ok_url],
+        policy="round_robin",
+        extra={
+            "retry_max_retries": 3,
+            "retry_initial_backoff_ms": 10,
+            "retry_max_backoff_ms": 50,
+        },
+    )
+
+    # A single request should succeed via retry to the healthy worker
+    r = requests.post(
+        f"{rh.url}/v1/completions",
+        json={
+            "model": "test-model",
+            "prompt": "crash",
+            "max_tokens": 1,
+            "stream": False,
+        },
+        timeout=5,
+    )
+    assert r.status_code == 200
+    # mock_workers fixture handles cleanup