Remove uniqueness requirement for district nces id (#652)

## What's changed?
- Removes uniqueness validation for `district_nces_id`

Author: jamiebenstead

app/models/school.rb

@@ -23,7 +23,6 @@ class School < ApplicationRecord
             format: { with: /\A\d{5,6}\z/, allow_nil: true, message: I18n.t('validations.school.reference') },
             if: :united_kingdom?
   validates :district_nces_id,
-            uniqueness: { conditions: -> { where(rejected_at: nil) }, case_sensitive: false, allow_blank: true, message: I18n.t('validations.school.district_nces_id_exists') },
             format: { with: /\A\d{7}\z/, allow_nil: true, message: I18n.t('validations.school.district_nces_id') },
             if: :united_states?
   validates :district_name, presence: true, if: :united_states?

config/locales/en.yml

@@ -18,7 +18,6 @@ en:
       reference: "must be 5-6 digits (e.g., 100000)"
       reference_urn_exists: "URN number already exists"
       district_nces_id: "must be 7 digits (e.g., 0100000)"
-      district_nces_id_exists: "NCES ID already exists"
       school_roll_number: "must be numbers followed by letters (e.g., 01572D)"
       school_roll_number_exists: "School roll number already exists"
     invitation:

db/migrate/20260116164502_remove_unique_index_from_schools_district_nces_id.rb

@@ -0,0 +1,5 @@
+class RemoveUniqueIndexFromSchoolsDistrictNcesId < ActiveRecord::Migration[7.2]
+  def change
+    remove_index :schools, name: "index_schools_on_district_nces_id"
+  end
+end

db/schema.rb

@@ -241,12 +241,6 @@
       expect(duplicate_school).not_to be_valid
     end
 
-    it 'returns error if district_nces_id is not unique' do
-      duplicate_school = build(:school, country_code: 'US', district_nces_id: '0100000')
-      duplicate_school.valid?
-      expect(duplicate_school.errors.details[:district_nces_id]).to include(hash_including(error: :taken))
-    end
-
     it 'accepts a valid district_nces_id format (7 digits)' do
       us_school.district_nces_id = '0100000'
       expect(us_school).to be_valid