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/conftest.py

@@ -1,5 +1,17 @@
 """Shared pytest configuration for integration tests."""
 import os
 import sys
+import pytest
 
 sys.path.insert(0, os.path.join(os.path.dirname(__file__), "integration"))
+
+from common import ServerManager
+
+
+@pytest.fixture(scope="session")
+def shared_server():
+    """One server for the entire test session. Avoids 27+ server start/stop cycles."""
+    server = ServerManager()
+    server.start()
+    yield server
+    server.stop()