fix(core-typings): add attachments field to MessageAttachmentDefault

The pin message attachment includes nested attachments but the type
did not declare it, causing AJV validation to reject it with
additionalProperties: false.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: ggazzo

packages/core-typings/src/IMessage/MessageAttachment/MessageAttachmentDefault.ts

@@ -1,5 +1,6 @@
 import type { Root } from '@rocket.chat/message-parser';
 
+import type { MessageAttachment } from './MessageAttachment';
 import type { MessageAttachmentBase } from './MessageAttachmentBase';
 
 export type MarkdownFields = 'text' | 'pretext' | 'fields';
@@ -32,4 +33,6 @@ export type MessageAttachmentDefault = {
 	thumb_url?: string;
 
 	color?: string;
+
+	attachments?: MessageAttachment[];
 } & MessageAttachmentBase;