fix: disable_ddl_transaction! on add_index action (#11833)

All migrations will automatically be wrapped in a transaction. There are
queries that you can’t execute inside a transaction. Adding index
concurrently is one of them, we have to disable the transaction. I
missed this in the earlier PR. #11831

db/migrate/20250627195529_add_index_to_messages.rb

@@ -1,4 +1,6 @@
 class AddIndexToMessages < ActiveRecord::Migration[7.0]
+  disable_ddl_transaction!
+
   def change
     # This index is added as a temporary fix for performance issues in the CSAT
     # responses controller where we query messages with account_id, content_type