fix(visualization, v1.2): take the union of Network Graph's two node columns

common/workflow-operator/src/main/scala/org/apache/texera/amber/operator/visualization/networkGraph/NetworkGraphOpDesc.scala

@@ -94,7 +94,9 @@ class NetworkGraphOpDesc extends PythonOperatorDescriptor {
          |        if not table.empty:
          |            sources = table[$source]
          |            destinations = table[$destination]
-         |            nodes = set(sources + destinations)
+         |            # Union of the two columns, in first-appearance order. Adding the
+         |            # Series pairs them off element-wise; a set reorders per run.
+         |            nodes = list(dict.fromkeys(pd.concat([sources, destinations]).tolist()))
          |            G = nx.Graph()
          |            for node in nodes:
          |                G.add_node(node)

common/workflow-operator/src/test/scala/org/apache/texera/amber/operator/visualization/networkGraph/NetworkGraphOpDescSpec.scala

@@ -0,0 +1,48 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+
+package org.apache.texera.amber.operator.visualization.networkGraph
+
+import org.scalatest.BeforeAndAfter
+import org.scalatest.flatspec.AnyFlatSpec
+import org.scalatest.matchers.should.Matchers
+
+class NetworkGraphOpDescSpec extends AnyFlatSpec with BeforeAndAfter with Matchers {
+
+  var opDesc: NetworkGraphOpDesc = _
+
+  before {
+    opDesc = new NetworkGraphOpDesc()
+  }
+
+  it should "build the node set as a union rather than by adding the two columns" in {
+    opDesc.source = "from_node"
+    opDesc.destination = "to_node"
+    val code = opDesc.generatePythonCode()
+
+    // `sources + destinations` is element-wise on two Series, so it glued each
+    // source to its destination and those strings entered the graph as nodes.
+    code should not include "set(sources + destinations)"
+    code should include("pd.concat([sources, destinations])")
+
+    // Ordered de-duplication, not a set: a set iterates strings in an order that
+    // varies between processes, which would move the nodes from run to run.
+    code should include("dict.fromkeys")
+  }
+}