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/v2/utils/draft.ts

@@ -0,0 +1,30 @@
+import type { IExecuteFunctions } from 'n8n-workflow';
+
+import type { IEmail } from '@utils/sendAndWait/interfaces';
+
+import { googleApiRequest } from '../../GenericFunctions';
+
+/**
+ * Adds inReplyTo and reference headers to the email if threadId is provided.
+ */
+export async function addThreadHeadersToEmail(
+	this: IExecuteFunctions,
+	email: IEmail,
+	threadId: string,
+): Promise<void> {
+	const thread = await googleApiRequest.call(
+		this,
+		'GET',
+		`/gmail/v1/users/me/threads/${threadId}`,
+		{},
+		{ format: 'metadata', metadataHeaders: ['Message-ID'] },
+	);
+
+	if (thread?.messages) {
+		const lastMessage = thread.messages.length - 1;
+		const messageId: string = thread.messages[lastMessage].payload.headers[0].value;
+
+		email.inReplyTo = messageId;
+		email.reference = messageId;
+	}
+}