fix mysql migration

db/migrate/20251121092044_add_is_first_to_completed_submitters.rb

@@ -9,6 +9,8 @@ class AddIsFirstToCompletedSubmitters < ActiveRecord::Migration[8.0]
     add_index :completed_submitters, %i[account_id completed_at],
               where: 'is_first = TRUE',
               name: 'index_completed_submitters_account_id_completed_at_is_first'
-    add_index :completed_submitters, :submission_id, unique: true, where: 'is_first = TRUE'
+
+    add_index :completed_submitters, :submission_id, unique: adapter_name != 'Mysql2',
+                                                     where: 'is_first = TRUE'
   end
 end