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

config/locales/en.yml

@@ -202,6 +202,8 @@ en:
     messages:
       instagram_story_content: '%{story_sender} mentioned you in the story: '
       instagram_deleted_story_content: This story is no longer available.
+      instagram_shared_story_content: 'Shared story'
+      instagram_shared_post_content: 'Shared post'
       deleted: This message was deleted
       whatsapp:
         list_button_label: 'Choose an item'