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.

packages/leann-core/src/leann/api.py

@@ -87,21 +87,23 @@ def compute_embeddings_via_server(chunks: list[str], model_name: str, port: int)
     # Connect to embedding server
     context = zmq.Context()
     socket = context.socket(zmq.REQ)
+    socket.setsockopt(zmq.LINGER, 0)  # Don't block on close
     socket.connect(f"tcp://localhost:{port}")
 
-    # Send chunks to server for embedding computation
-    request = chunks
-    socket.send(msgpack.packb(request))
+    try:
+        # Send chunks to server for embedding computation
+        request = chunks
+        socket.send(msgpack.packb(request))
 
-    # Receive embeddings from server
-    response = socket.recv()
-    embeddings_list = msgpack.unpackb(response)
+        # Receive embeddings from server
+        response = socket.recv()
+        embeddings_list = msgpack.unpackb(response)
 
-    # Convert back to numpy array
-    embeddings = np.array(embeddings_list, dtype=np.float32)
-
-    socket.close()
-    context.term()
+        # Convert back to numpy array
+        embeddings = np.array(embeddings_list, dtype=np.float32)
+    finally:
+        socket.close(linger=0)
+        context.term()
 
     return embeddings