Refactor ActiveRecord::SignedId to not rely on relation delegation

Ref: rails#50396
Ref: rails#51776

`ActiveRecord::Relation` automatically delegates missing methods
to the model class wrapped in a `scoping { }` block.

This is to support scoping in user defined class methods. The problem
however is that it's very error prone for the framework, because we
can mistakenly call model methods from inside `Relation` and not
realized we're applying a global scope.

In the best case scenario it's just a waste of performance, but
it can also lead to bugs like rails#51775

I'm planning to restrict this automatic delegation to methods defined
in childs of `ActiveRecord::Base` only: rails#50396
but for this to work we must first refactor any Rails code that rely on it.

Author: byroot

activerecord/lib/active_record/relation.rb

@@ -66,6 +66,7 @@ def exec_explain(&block)
 
     include Enumerable
     include FinderMethods, Calculations, SpawnMethods, QueryMethods, Batches, Explain, Delegation
+    include SignedId::RelationMethods
 
     attr_reader :table, :klass, :loaded, :predicate_builder
     attr_accessor :skip_preloading_value

activerecord/lib/active_record/signed_id.rb

@@ -13,6 +13,16 @@ module SignedId
       class_attribute :signed_id_verifier_secret, instance_writer: false
     end
 
+    module RelationMethods # :nodoc:
+      def find_signed(...)
+        scoping { model.find_signed(...) }
+      end
+
+      def find_signed!(...)
+        scoping { model.find_signed!(...) }
+      end
+    end
+
     module ClassMethods
       # Lets you find a record based on a signed id that's safe to put into the world without risk of tampering.
       # This is particularly useful for things like password reset or email verification, where you want