fix(Gmail Node): Do not break threads while creating drafts (#16272)

Co-authored-by: Shireen Missi <94372015+ShireenMissi@users.noreply.github.com>
Co-authored-by: cubic-dev-ai[bot] <191113872+cubic-dev-ai[bot]@users.noreply.github.com>

packages/nodes-base/nodes/Google/Gmail/test/v2/utils.test.ts

@@ -2,7 +2,11 @@ import { mock } from 'jest-mock-extended';
 import { DateTime } from 'luxon';
 import type { IExecuteFunctions, INode } from 'n8n-workflow';
 
+import type { IEmail } from '@utils/sendAndWait/interfaces';
+
+import * as GenericFunctions from '../../GenericFunctions';
 import { parseRawEmail, prepareTimestamp } from '../../GenericFunctions';
+import { addThreadHeadersToEmail } from '../../v2/utils/draft';
 
 const node: INode = {
 	id: '1',
@@ -128,3 +132,73 @@ describe('parseRawEmail', () => {
 		expect(typeof json.date).toBe('string');
 	});
 });
+
+describe('addThreadHeadersToEmail', () => {
+	beforeEach(() => {
+		jest.clearAllMocks();
+	});
+
+	it('should set inReplyTo and reference on the email object', async () => {
+		const mockThreadId = 'thread123';
+		const mockMessageId = '<message-id@example.com>';
+		const mockThread = {
+			messages: [
+				{ payload: { headers: [{ value: '<old-id@example.com>' }] } },
+				{ payload: { headers: [{ value: mockMessageId }] } },
+			],
+		};
+
+		jest.spyOn(GenericFunctions, 'googleApiRequest').mockImplementation(async function () {
+			return mockThread;
+		});
+
+		const email = mock<IEmail>({});
+
+		const thisArg = mock<IExecuteFunctions>({});
+
+		await addThreadHeadersToEmail.call(thisArg, email, mockThreadId);
+
+		expect(email.inReplyTo).toBe(mockMessageId);
+		expect(email.reference).toBe(mockMessageId);
+	});
+
+	it('should set inReplyTo and reference on the email object even if the message has only one item', async () => {
+		const mockThreadId = 'thread123';
+		const mockMessageId = '<message-id@example.com>';
+		const mockThread = {
+			messages: [{ payload: { headers: [{ value: mockMessageId }] } }],
+		};
+
+		jest.spyOn(GenericFunctions, 'googleApiRequest').mockImplementation(async function () {
+			return mockThread;
+		});
+
+		const email = mock<IEmail>({});
+
+		const thisArg = mock<IExecuteFunctions>({});
+
+		await addThreadHeadersToEmail.call(thisArg, email, mockThreadId);
+
+		expect(email.inReplyTo).toBe(mockMessageId);
+		expect(email.reference).toBe(mockMessageId);
+	});
+
+	it('should not do anything if the thread has no messages', async () => {
+		const mockThreadId = 'thread123';
+		const mockThread = {};
+
+		jest.spyOn(GenericFunctions, 'googleApiRequest').mockImplementation(async function () {
+			return mockThread;
+		});
+
+		const email = mock<IEmail>({});
+
+		const thisArg = mock<IExecuteFunctions>({});
+
+		await addThreadHeadersToEmail.call(thisArg, email, mockThreadId);
+
+		// We are using mock<IEmail>({}) which means the value of these will be a mock function
+		expect(typeof email.inReplyTo).toBe('function');
+		expect(typeof email.reference).toBe('function');
+	});
+});