refactor(core): Consolidate task result validation in Code node (#18928)

packages/nodes-base/nodes/Code/reserved-key-found-error.ts

@@ -0,0 +1,11 @@
+import { ValidationError } from './ValidationError';
+
+export class ReservedKeyFoundError extends ValidationError {
+	constructor(reservedKey: string, itemIndex: number) {
+		super({
+			message: 'Invalid output format',
+			description: `An output item contains the reserved key <code>${reservedKey}</code>. To get around this, please wrap each item in an object, under a key called <code>json</code>. <a href="https://docs.n8n.io/data/data-structure/#data-structure" target="_blank">Example</a>`,
+			itemIndex,
+		});
+	}
+}