Fix CurrentAttributes not setting an attribute's default:

- Fix rails#54636
- ### Problem

  When calling an attribute that has a default, `nil` would be returned
  instead.

  ### Context

  CurrentAttributes uses a cache to store each CurrentAttributes
  subclasses instance. When the class method `instance` gets
  called, we instantiate the class and populate the cache.
  The defaults of each attributes are set when the class is
  instantiated.

  This basically means that once we instantiate the class, all
  attribute having a default would be ignored.

  ### Details

  CurrentAttributes has a `method_added` hook which also call the
  `instance` class method. So depending on how your class is setup
  (e.g. you define a method and then define an attribute with a
  default), you'd hit the bug.

  This is even more sneaky when using a `delgate_to`:

  ```ruby
  class Current < CurrentAttributes
    attribute :user_session
    delegate :user, to: :user_session # Ends up triggering the `method_added` hook

    attribute :foo, default: 0 # Default would be ignored.
  end
  ```

  ### Solution

  CurrentAttribute: avoid creating the instance in `method_added` hook.

Co-authored-by: Jean Boussier <[email protected]>

Author: Edouard-chin

activesupport/lib/active_support/current_attributes.rb

@@ -187,7 +187,7 @@ def method_added(name)
           super
           return if name == :initialize
           return unless public_method_defined?(name)
-          return if respond_to?(name, true)
+          return if singleton_class.method_defined?(name) || singleton_class.private_method_defined?(name)
           Delegation.generate(singleton_class, [name], to: :instance, as: self, nilable: false)
         end
     end

activesupport/test/current_attributes_test.rb

@@ -262,4 +262,18 @@ class InvalidAttributeNames < ActiveSupport::CurrentAttributes
       end
     end
   end
+
+  test "method_added hook doesn't reach the instance. Fix for #54646" do
+    current = Class.new(ActiveSupport::CurrentAttributes) do
+      def self.name
+        "MyCurrent"
+      end
+
+      def foo; end # Sets the cache because of a `method_added` hook
+
+      attribute :bar, default: {}
+    end
+
+    assert_instance_of(Hash, current.bar)
+  end
 end