tests: share server across tests, suppress probe noise

- Add session-scoped shared_server fixture to avoid 27+ server start/stop cycles
- Refactor test_tcp.py and test_features.py to use shared server
- Suppress server health-check probe stderr noise in common.py

tests/integration/common.py

@@ -28,7 +28,7 @@ class ServerManager:
         cmd = SERVER_CMD + ["-p", str(self._port)]
         if extra_args:
             cmd += extra_args
-        self._proc = subprocess.Popen(cmd, stdout=subprocess.DEVNULL, stderr=None)
+        self._proc = subprocess.Popen(cmd, stdout=subprocess.DEVNULL, stderr=subprocess.DEVNULL)
         _wait_for_port(self._port, timeout=5)
 
     def stop(self):