feat: Add support for shared post and story attachment types in Instagram messages (#12997)

When users share Instagram posts or stories via DM, Instagram sends
webhooks with type `ig_post` and `ig_story` attachments. The system was
failing on these types because they weren't defined in the file_types.
This PR fixes the issue by handling all shared types and rendering them
on the front end.

**Shared post**

<img width="2154" height="1828" alt="CleanShot 2025-12-03 at 16 29
14@2x"
src="https://github.com/user-attachments/assets/7e731171-4904-43a6-abeb-b1db2c262742"
/>

**Shared status**
<img width="1702" height="1676" alt="CleanShot 2025-12-03 at 16 10
25@2x"
src="https://github.com/user-attachments/assets/6a151233-ce47-429d-b7c2-061514b20e05"
/>


Fixes
https://linear.app/chatwoot/issue/CW-5441/argumenterror-ig-story-is-not-a-valid-file-type-argumenterror

app/javascript/dashboard/components-next/message/constants.js

@@ -49,6 +49,8 @@ export const ATTACHMENT_TYPES = {
   STORY_MENTION: 'story_mention',
   CONTACT: 'contact',
   IG_REEL: 'ig_reel',
+  IG_POST: 'ig_post',
+  IG_STORY: 'ig_story',
 };
 
 export const CONTENT_TYPES = {