[drupalMedia] Media previews do not update after alt text was modified

Problem/Motivation

Discovered while working on #3206522: Add FunctionalJavascript test coverage for media library.

Steps to reproduce

Override alt for some <drupal-media>
I expect this to trigger a XHR to in the preview show the updated alt

But this does not happen.

I already spent ~45 mins debugging this and searching the CKEditor 5 docs, unsuccessfully. The editingDowncast in drupalmediatoolbar.js is supposed to be invoked whenever the model is changed, but that is definitely not happening.

Perhaps this means that we're setting the model attributes incorrectly?

Proposed resolution

TBD

Remaining tasks

TBD

User interface changes

None.

API changes

None.

Data model changes

None.