Invoke load_schema in _default_attributes

Currently `_default_attributes` doesn't work unless `load_schema` is
called before.

The `MissingAttributeError` is caused by `reload_schema_from_cache` is
invoked by `serialize`.

I added `load_schema` in `_default_attributes` to `_default_attributes`
works without any dependency like `attribute_types` etc.

Closes #31905.

activerecord/lib/active_record/model_schema.rb

@@ -379,6 +379,7 @@ def column_defaults
       end
 
       def _default_attributes # :nodoc:
+        load_schema
         @default_attributes ||= ActiveModel::AttributeSet.new({})
       end
 

activerecord/test/cases/persistence_test.rb

@@ -251,6 +251,13 @@ def test_becomes
     assert_equal "The First Topic", topics(:first).becomes(Reply).title
   end
 
+  def test_becomes_after_reload_schema_from_cache
+    Reply.define_attribute_methods
+    Reply.serialize(:content) # invoke reload_schema_from_cache
+    assert_kind_of Reply, topics(:first).becomes(Reply)
+    assert_equal "The First Topic", topics(:first).becomes(Reply).title
+  end
+
   def test_becomes_includes_errors
     company = Company.new(name: nil)
     assert_not_predicate company, :valid?