fix: cw-5411 handle unrepresentable image attachments (#12178)

# Pull Request Template

## Description

Fixes
https://linear.app/chatwoot/issue/CW-5411/actionviewtemplateerror-activestorageunrepresentableerror

###  Problem
API endpoints return 500 errors when conversations contain image
attachments that can't be processed by ActiveStorage (e.g., files with
non-ASCII filenames, corrupted images, or malicious XSS filenames).

Root Cause: Commit 6cab74139 removed the representable? safety check
from thumb_url, causing `ActiveStorage::UnrepresentableError` to bubble
up and crash the API when it encountered a malformed image file.

Fix: Rescue `thumb_url` method to catch UnrepresentableError and return
an empty string while logging problematic names for future debugging.

This ensures the messages/attachments api does not break due to a single
corrupted image file.

## Type of change

- [x] Bug fix (non-breaking change which fixes an issue)

## How Has This Been Tested?

- Added specs

## Checklist:

- [x] My code follows the style guidelines of this project
- [x] I have performed a self-review of my code
- [x] I have commented on my code, particularly in hard-to-understand
areas
- [x] I have made corresponding changes to the documentation
- [x] My changes generate no new warnings
- [x] I have added tests that prove my fix is effective or that my
feature works
- [x] New and existing unit tests pass locally with my changes
- [x] Any dependent changes have been merged and published in downstream
modules

app/models/attachment.rb

@@ -62,7 +62,12 @@ class Attachment < ApplicationRecord
   def thumb_url
     return '' unless file.attached? && image?
 
-    url_for(file.representation(resize_to_fill: [250, nil]))
+    begin
+      url_for(file.representation(resize_to_fill: [250, nil]))
+    rescue ActiveStorage::UnrepresentableError => e
+      Rails.logger.warn "Unrepresentable image attachment: #{id} (#{file.filename}) - #{e.message}"
+      ''
+    end
   end
 
   def with_attached_file?

spec/models/attachment_spec.rb

@@ -82,6 +82,16 @@ RSpec.describe Attachment do
 
       expect(attachment.thumb_url).to be_present
     end
+
+    it 'handles unrepresentable images gracefully' do
+      attachment = message.attachments.create!(account_id: message.account_id, file_type: :image)
+      attachment.file.attach(io: StringIO.new('fake image'), filename: 'test.jpg', content_type: 'image/jpeg')
+
+      allow(attachment.file).to receive(:representation).and_raise(ActiveStorage::UnrepresentableError.new('Cannot represent'))
+
+      expect(Rails.logger).to receive(:warn).with(/Unrepresentable image attachment: #{attachment.id}/)
+      expect(attachment.thumb_url).to eq('')
+    end
   end
 
   describe 'meta data handling' do