fix: prevent test runner hanging on Python 3.9/3.13 due to ZMQ and process cleanup issues

Based on excellent analysis from user, implemented comprehensive fixes:

1. ZMQ Socket Cleanup:
   - Set LINGER=0 on all ZMQ sockets (client and server)
   - Use try-finally blocks to ensure socket.close() and context.term()
   - Prevents blocking on exit when ZMQ contexts have pending operations

2. Global Test Cleanup:
   - Added tests/conftest.py with session-scoped cleanup fixture
   - Cleans up leftover ZMQ contexts and child processes after all tests
   - Lists remaining threads for debugging

3. CI Improvements:
   - Apply timeout to ALL Python versions on Linux (not just 3.13)
   - Increased timeout to 180s for better reliability
   - Added process cleanup (pkill) on timeout

4. Dependencies:
   - Added psutil>=5.9.0 to test dependencies for process management

Root cause: Python 3.9/3.13 are more sensitive to cleanup timing during
interpreter shutdown. ZMQ's default LINGER=-1 was blocking exit, and
atexit handlers were unreliable for cleanup.

This should resolve the 'all tests pass but CI hangs' issue.

.github/workflows/build-reusable.yml

@@ -263,25 +263,25 @@ jobs:
           # Activate virtual environment
           source .venv/bin/activate || source .venv/Scripts/activate
 
-          # Run all tests with timeout to debug Python 3.13 hanging issue
-          # Using timeout with INT signal to get full traceback when hanging (Linux only)
-          if [[ "${{ matrix.python }}" == "3.13" ]] && [[ "$RUNNER_OS" == "Linux" ]]; then
-            echo "Running tests with timeout for Python 3.13 debugging (Linux)..."
-            timeout --signal=INT 120 pytest tests/ --full-trace -v || {
+          # Run all tests with timeout on Linux to prevent hanging
+          if [[ "$RUNNER_OS" == "Linux" ]]; then
+            echo "Running tests with timeout (Linux)..."
+            timeout --signal=INT 180 pytest tests/ -v || {
               EXIT_CODE=$?
               if [ $EXIT_CODE -eq 124 ]; then
-                echo "⚠️ Tests timed out after 120 seconds - likely hanging during collection"
-                echo "This is a known issue with Python 3.13 - see traceback above for details"
+                echo "⚠️ Tests timed out after 180 seconds - likely process cleanup issue"
+                echo "Check for lingering ZMQ connections or child processes"
+                # Try to clean up any leftover processes
+                pkill -TERM -P $$ || true
+                sleep 1
+                pkill -KILL -P $$ || true
               fi
               exit $EXIT_CODE
             }
-          elif [[ "${{ matrix.python }}" == "3.13" ]]; then
-            # For macOS/Windows, run with verbose output but no timeout
-            echo "Running tests for Python 3.13 (no timeout on $RUNNER_OS)..."
-            pytest tests/ --full-trace -v
           else
-            # Normal test run for other Python versions
-            pytest tests/
+            # For macOS/Windows, run without GNU timeout
+            echo "Running tests ($RUNNER_OS)..."
+            pytest tests/ -v
           fi
 
       - name: Run sanity checks (optional)