[CI] Fail subprocess tests with root-cause error (#23795)

Signed-off-by: Nick Hill <nhill@redhat.com>

tests/async_engine/test_api_server.py

@@ -1,6 +1,7 @@
 # SPDX-License-Identifier: Apache-2.0
 # SPDX-FileCopyrightText: Copyright contributors to the vLLM project
 
+import copyreg
 import os
 import subprocess
 import sys
@@ -10,6 +11,30 @@ from pathlib import Path
 
 import pytest
 import requests
+import urllib3.exceptions
+
+
+def _pickle_new_connection_error(obj):
+    """Custom pickler for NewConnectionError to fix tblib compatibility."""
+    # Extract the original message by removing the "conn: " prefix
+    full_message = obj.args[0] if obj.args else ""
+    if ': ' in full_message:
+        # Split off the connection part and keep the actual message
+        _, actual_message = full_message.split(': ', 1)
+    else:
+        actual_message = full_message
+    return _unpickle_new_connection_error, (actual_message, )
+
+
+def _unpickle_new_connection_error(message):
+    """Custom unpickler for NewConnectionError."""
+    # Create with None as conn and the actual message
+    return urllib3.exceptions.NewConnectionError(None, message)
+
+
+# Register the custom pickle/unpickle functions for tblib compatibility
+copyreg.pickle(urllib3.exceptions.NewConnectionError,
+               _pickle_new_connection_error)
 
 
 def _query_server(prompt: str, max_tokens: int = 5) -> dict:
@@ -52,6 +77,7 @@ def api_server(distributed_executor_backend: str):
     uvicorn_process.terminate()
 
 
+@pytest.mark.timeout(300)
 @pytest.mark.parametrize("distributed_executor_backend", ["mp", "ray"])
 def test_api_server(api_server, distributed_executor_backend: str):
     """