Fix unprepared_statement to work it when nesting

I've been reported a problem by @osyo-manga, preloading queries in
`unprepared_statement` doesn't work as expected in main branch (queries
are executed as prepared statements).

It is caused by #41385, due to `scope.to_sql` in `grouping_key` depends
on `unprepared_statement` which has an issue when nesting.

To fix the issue, don't add/delete object_id in the prepared statements
disabled cache if that is already disabled.

activerecord/lib/active_record/connection_adapters/abstract_adapter.rb

@@ -160,9 +160,10 @@ def schema_migration # :nodoc:
                               end
       end
 
-      def prepared_statements
+      def prepared_statements?
         @prepared_statements && !prepared_statements_disabled_cache.include?(object_id)
       end
+      alias :prepared_statements :prepared_statements?
 
       def prepared_statements_disabled_cache # :nodoc:
         Thread.current[:ar_prepared_statements_disabled_cache] ||= Set.new
@@ -256,7 +257,7 @@ def seconds_idle # :nodoc:
       end
 
       def unprepared_statement
-        cache = prepared_statements_disabled_cache.add(object_id) if @prepared_statements
+        cache = prepared_statements_disabled_cache.add?(object_id) if @prepared_statements
         yield
       ensure
         cache&.delete(object_id)

activerecord/test/cases/bind_parameter_test.rb

@@ -173,6 +173,22 @@ def test_bind_params_to_sql_with_unprepared_statements
         end
       end
 
+      def test_nested_unprepared_statements
+        assert_predicate @connection, :prepared_statements?
+
+        @connection.unprepared_statement do
+          assert_not_predicate @connection, :prepared_statements?
+
+          @connection.unprepared_statement do
+            assert_not_predicate @connection, :prepared_statements?
+          end
+
+          assert_not_predicate @connection, :prepared_statements?
+        end
+
+        assert_predicate @connection, :prepared_statements?
+      end
+
       private
         def assert_bind_params_to_sql
           table = Author.quoted_table_name