refactor(type de champ): extract codes_and_names method

app/controllers/administrateurs/groupe_instructeurs_controller.rb

@@ -42,7 +42,7 @@ module Administrateurs
 
       tdc_options = case tdc.type_champ
       when TypeDeChamp.type_champs.fetch(:departements)
-        APIGeoService.departements.map { "#{_1[:code]} – #{_1[:name]}" }
+        tdc.codes_and_names
       when TypeDeChamp.type_champs.fetch(:drop_down_list)
         tdc.drop_down_options.reject(&:empty?)
       end

app/models/groupe_instructeur.rb

@@ -108,7 +108,7 @@ class GroupeInstructeur < ApplicationRecord
 
     options = case routing_tdc.type_champ
     when TypeDeChamp.type_champs.fetch(:departements)
-      APIGeoService.departements.map { "#{_1[:code]} – #{_1[:name]}" }
+      routing_tdc.codes_and_names
     when TypeDeChamp.type_champs.fetch(:drop_down_list)
       routing_tdc.options_with_drop_down_other
     end

app/models/type_de_champ.rb

@@ -477,6 +477,12 @@ class TypeDeChamp < ApplicationRecord
     end
   end
 
+  def codes_and_names
+    if departement?
+      APIGeoService.departements.map { "#{_1[:code]} – #{_1[:name]}" }
+    end
+  end
+
   # historicaly we added a blank ("") option by default to avoid wrong selection
   #   see self.parse_drop_down_list_value
   #   then rails decided to add this blank ("") option when the select is required