fix: Prepend UTF-8 BOM to contact CSV export for non-ASCII character support (#14123)

## Description

Spreadsheet applications such as Microsoft Excel do not auto-detect
UTF-8 encoding when opening CSV files. This causes non-ASCII characters
(Arabic, Japanese, Chinese, Korean, etc.) to appear garbled in the
exported contacts CSV.

This PR prepends the UTF-8 Byte Order Mark (`EF BB BF`) to the CSV
output in `Account::ContactsExportJob`, which signals to spreadsheet
applications that the file is UTF-8 encoded.

Fixes: #13998

app/jobs/account/contacts_export_job.rb

@@ -42,8 +42,13 @@ class Account::ContactsExportJob < ApplicationJob
   def attach_export_file(csv_data)
     return if csv_data.blank?
 
+    # Prepend UTF-8 BOM so that spreadsheet applications (e.g. Excel)
+    # correctly recognise the file encoding for non-ASCII characters
+    # such as Arabic, Japanese, and Chinese.
+    bom = "\xEF\xBB\xBF"
+
     @account.contacts_export.attach(
-      io: StringIO.new(csv_data),
+      io: StringIO.new("#{bom}#{csv_data}"),
       filename: "#{@account.name}_#{@account.id}_contacts.csv",
       content_type: 'text/csv'
     )

spec/jobs/account/contacts_export_job_spec.rb

@@ -85,6 +85,13 @@ RSpec.describe Account::ContactsExportJob do
       expect(phone_numbers).to include('+910808080818', '+910808080808')
     end
 
+    it 'prepends UTF-8 BOM to the exported CSV for spreadsheet compatibility' do
+      described_class.perform_now(account.id, user.id, [], {})
+
+      raw = account.contacts_export.download
+      expect(raw.bytes[0..2]).to eq([0xEF, 0xBB, 0xBF])
+    end
+
     it 'returns all resolved contacts as results when filter is not prvoided' do
       create(:contact, account: account, email: nil, phone_number: nil)
       described_class.perform_now(account.id, user.id, %w[id name email column_not_present], {})