fix: scope external_url override to Instagram DM conversations only (#13982)

Previously, all incoming messages from Facebook channel with
instagram_id had their attachment data_url and thumb_url overridden with
external_url. This caused issues for non-Instagram conversations
originating from Facebook Message where the file URL should be used
instead.

Narrows the override to only apply when the conversation type is
instagram_direct_message, which is the only case where Instagram's CDN
URLs need to be used directly.

Fixes
https://linear.app/chatwoot/issue/CW-6722/videos-are-missing-in-facebook-conversation

---------

Co-authored-by: Muhsin <12408980+muhsin-k@users.noreply.github.com>
Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

app/models/attachment.rb

@@ -120,7 +120,7 @@ class Attachment < ApplicationRecord
       height: file.metadata[:height]
     }
 
-    metadata[:data_url] = metadata[:thumb_url] = external_url if message.inbox.instagram? && message.incoming?
+    metadata[:data_url] = metadata[:thumb_url] = external_url if instagram_incoming_message?
     metadata
   end
 
@@ -156,6 +156,14 @@ class Attachment < ApplicationRecord
     }
   end
 
+  def instagram_incoming_message?
+    return false unless message.incoming?
+
+    return true if message.inbox.instagram_direct?
+
+    message.inbox.instagram? && message.conversation&.additional_attributes&.dig('type') == 'instagram_direct_message'
+  end
+
   def set_extension
     return unless file.attached?
     return if extension.present?