fix(core): Serialize BigInts (#6805)

* workaround for non-serializable BigInt

* refactor

packages/workflow/src/types.d.ts

@@ -12,3 +12,7 @@ declare module '@n8n_io/riot-tmpl' {
 	let brackets: Brackets;
 	let tmpl: Tmpl;
 }
+
+interface BigInt {
+	toJSON(): string;
+}

packages/workflow/src/utils.ts

@@ -3,6 +3,12 @@ import type { BinaryFileType, JsonObject } from './Interfaces';
 
 const readStreamClasses = new Set(['ReadStream', 'Readable', 'ReadableStream']);
 
+// NOTE: BigInt.prototype.toJSON is not available, which causes JSON.stringify to throw an error
+// as well as the flatted stringify method. This is a workaround for that.
+BigInt.prototype.toJSON = function () {
+	return this.toString();
+};
+
 export const isObjectEmpty = (obj: object | null | undefined): boolean => {
 	if (obj === undefined || obj === null) return true;
 	if (typeof obj === 'object') {