FIX: skip category preference update if already set by group. (#20823)

`default_categories_*` site settings will update the category preferences on user creation. But it shouldn't update the user's category preference if a group's setting already updated it for that user.

app/models/user.rb

@@ -1968,7 +1968,7 @@ class User < ActiveRecord::Base
       end
     end
 
-    CategoryUser.insert_all!(values) if values.present?
+    CategoryUser.insert_all(values) if values.present?
   end
 
   def set_default_tags_preferences

spec/models/user_spec.rb

@@ -2097,6 +2097,19 @@ RSpec.describe User do
       expect(CategoryUser.lookup(user, :regular).pluck(:category_id)).to eq([category4.id])
     end
 
+    it "does not update category preferences if already set by group" do
+      user_id = 123
+
+      CategoryUser.create!(
+        user_id: user_id,
+        category_id: category2.id,
+        notification_level: CategoryUser.notification_levels[:normal],
+      )
+      user = Fabricate(:user, id: user_id, trust_level: 1)
+
+      expect(CategoryUser.lookup(user, :normal).pluck(:category_id)).to include(category2.id)
+    end
+
     it "does not set category preferences for staged users" do
       user = Fabricate(:user, staged: true)
       expect(CategoryUser.lookup(user, :watching).pluck(:category_id)).to eq([])