fix: added a missing test, corrected the imports, added behavior for disconnected goals as last step

ref: N25B-434

src/pages/VisProgPage/visualProgrammingUI/components/Plan.tsx

@@ -1,5 +1,6 @@
-import { GoalReduce } from "../nodes/GoalNode"
 import { type Node } from "@xyflow/react"
+import { GoalReduce } from "../nodes/GoalNode"
+
 
 export type Plan = {
     name: string,

src/pages/VisProgPage/visualProgrammingUI/components/PlanEditingFunctions.tsx

@@ -27,8 +27,9 @@ export function insertGoalInPlan(plan: Plan, goalNode: GoalNode): Plan {
  * @param goalID: the goal node to delete.
  * @returns: a new plan with the goal removed.
  */
-export function deleteGoalInPlanByID(plan: Plan, goalID: string): Plan {
-    return {...plan,
+export function deleteGoalInPlanByID(plan: Plan, goalID: string) {
+    const updatedPlan = {...plan,
         steps: plan.steps.filter((x) => x.id !== goalID)
     }
+    return updatedPlan.steps.length == 0 ? undefined : updatedPlan
 }