Eager load the association classes on boot

We were expecting those classes to be loaded when an association is
defined but they are not.

If you add a debugger on the first line of any of those classes and try
to define the corresponding association you will see that the classes
are not loaded.

Fixes #26273

activerecord/lib/active_record/associations.rb

@@ -224,13 +224,6 @@ module Associations # :nodoc:
     autoload :CollectionAssociation
     autoload :ForeignAssociation
     autoload :CollectionProxy
-
-    autoload :BelongsToAssociation
-    autoload :BelongsToPolymorphicAssociation
-    autoload :HasManyAssociation
-    autoload :HasManyThroughAssociation
-    autoload :HasOneAssociation
-    autoload :HasOneThroughAssociation
     autoload :ThroughAssociation
 
     module Builder #:nodoc:
@@ -245,6 +238,13 @@ module Builder #:nodoc:
     end
 
     eager_autoload do
+      autoload :BelongsToAssociation
+      autoload :BelongsToPolymorphicAssociation
+      autoload :HasManyAssociation
+      autoload :HasManyThroughAssociation
+      autoload :HasOneAssociation
+      autoload :HasOneThroughAssociation
+
       autoload :Preloader
       autoload :JoinDependency
       autoload :AssociationScope