H5P button is not available for atto editor when using textarea customfield

[TomoTsuyuki]

H5P button is not available for atto editor when using textarea customfield

How to replicate:

• Create a cms content type with textarea customfield
• Create a course
• Create a cms content using the cms content type with text area customfield
• Enrol a teacher
• Login by the teacher account (not using "login as" function)
• Edit the cms activity
• The customfield editor doesn't have H5P button for the teacher. (It's available for admin)

The issue is the text editor (TinyMCE / Atto) is loaded by the system context, which teacher role doesn't have capability for loading H5P button.

Textarea using $this->get_handler()->get_configuration_context();, which returns system context.
https://github.com/moodle/moodle/blob/ec7711b9a60402891db61357f914be161bc3dfbe/customfield/field/textarea/classes/field_controller.php#L115

moodle-mod_cms/classes/customfield/cmsfield_handler.php

Line 41 in e284319

return \context_system::instance();

The context should be from instance context, not system context.