FEATURE: add system_user_max_attachment_size_kb site setting (#28351)

* System user attachment size WIP

* spec check

* controller update

* add max to system_user_max_attachment_size_kb

* DEV: update to use static method for `max_attachment_size_for_user`

add test to use large image.
add check for failure.

* DEV: update `system_user_max_attachment_size_kb` default value to 0

remove unecessary test.
update tests to reflect the new default value of `system_user_max_attachment_size_kb`

* DEV: update maximum_file_size to check when is an attachment made by a system user

Add tests for when `system_user_max_attachment_size_kb` is over and under the limit
Add test for checking interaction with `max_attachment_size_kb`

* DEV: move `max_attachment_size_for_user` to private methods

* DEV: turn `max_attachment_size_for_user` into a static method

* DEV: typo in test case

* DEV: move max_attachment_size_for_user to private class method

* Revert "DEV: move max_attachment_size_for_user to private class method"

This reverts commit 5d5ae0b715de7c3453dc1392df299ef3bb58990c.

---------

Co-authored-by: Gabriel Grubba <gabriel@discourse.org>

lib/validators/upload_validator.rb

@@ -146,7 +146,14 @@ class UploadValidator < ActiveModel::Validator
       if upload.for_export
         SiteSetting.max_export_file_size_kb
       else
-        SiteSetting.get("max_#{type}_size_kb")
+        if upload.user&.id == Discourse::SYSTEM_USER_ID && type == "attachment"
+          [
+            SiteSetting.get("system_user_max_attachment_size_kb"),
+            SiteSetting.get("max_attachment_size_kb"),
+          ].max
+        else
+          SiteSetting.get("max_#{type}_size_kb")
+        end
       end
 
     max_size_bytes = max_size_kb.kilobytes