Prevent CurrentAttributes defaults from leaking

Follow-up to #50677.

Prior to this commit, all `ActiveSupport::CurrentAttributes` subclasses
stored their default values in the same `Hash`, causing default values
to leak between classes.  This commit ensures each subclass maintains a
separate `Hash`.

This commit also simplifies the resolution of default values, replacing
the `merge_defaults!` method with `resolve_defaults`.

activesupport/lib/active_support/current_attributes.rb

@@ -134,7 +134,7 @@ def attribute(*names, default: nil)
 
         singleton_class.delegate(*names.flat_map { |name| [name, "#{name}="] }, to: :instance, as: self)
 
-        defaults.merge! names.index_with { default }
+        self.defaults = defaults.merge(names.index_with { default })
       end
 
       # Calls this callback before #reset is called on the instance. Used for resetting external collaborators that depend on current values.
@@ -188,12 +188,12 @@ def method_added(name)
         end
     end
 
-    class_attribute :defaults, instance_writer: false, default: {}
+    class_attribute :defaults, instance_writer: false, default: {}.freeze
 
     attr_accessor :attributes
 
     def initialize
-      @attributes = merge_defaults!({})
+      @attributes = resolve_defaults
     end
 
     # Expose one or more attributes within a block. Old values are returned after the block concludes.
@@ -213,20 +213,14 @@ def set(attributes, &block)
     # Reset all attributes. Should be called before and after actions, when used as a per-request singleton.
     def reset
       run_callbacks :reset do
-        self.attributes = merge_defaults!({})
+        self.attributes = resolve_defaults
       end
     end
 
     private
-      def merge_defaults!(attributes)
-        defaults.each_with_object(attributes) do |(name, default), values|
-          value =
-            case default
-            when Proc then default.call
-            else default.dup
-            end
-
-          values[name] = value
+      def resolve_defaults
+        defaults.transform_values do |value|
+          Proc === value ? value.call : value.dup
         end
       end
   end

activesupport/test/current_attributes_test.rb

@@ -214,6 +214,13 @@ def after_teardown
     assert_equal true, Current.respond_to?("respond_to_test")
   end
 
+  test "CurrentAttributes defaults do not leak between classes" do
+    Class.new(ActiveSupport::CurrentAttributes) { attribute :counter_integer, default: 100 }
+    Current.reset
+
+    assert_equal 0, Current.counter_integer
+  end
+
   test "CurrentAttributes use fiber-local variables" do
     previous_level = ActiveSupport::IsolatedExecutionState.isolation_level
     ActiveSupport::IsolatedExecutionState.isolation_level = :fiber