test: add vitest baseline for critical payment and auth guards

convex/ai-utils.ts

@@ -0,0 +1,9 @@
+type NodeCanvasRef = {
+  canvasId: string;
+};
+
+export function assertNodeBelongsToCanvasOrThrow(node: NodeCanvasRef, canvasId: string): void {
+  if (node.canvasId !== canvasId) {
+    throw new Error("Node does not belong to canvas");
+  }
+}