Psalm found more than 20 potential problems in the proposed changes. Check the Files changed tab for more details.

Yay, it looks gooood. Didn't make a full review yet, just using it for the work on the assistant.
It breaks NC though. I tried to rebase on master, no luck.

PHP Fatal error:  Class OC\\Files\\Node\\LazyR
oot contains 1 abstract method and must therefore be declared abstract or implement the remaining methods (OCP\\Files\\IRootFolder::getAppDataDirectoryName) in /var/www/html/dev
/server/lib/private/Files/Node/LazyRoot.php on line 39

A bump of the server version is needed to trigger the migration step.

Another partial review:

• We could use (optional) default values for input shape fields. It might not make sense in many cases but for example: "number of images" for TextToImage could have 1 as default value. Wdyt?
• The assistant needs a manager method AND/OR an endpoint to get all user's tasks (with a task type ID optional filter). Wdyt?
• [minor] CSRF might not be needed for taskprocessing/tasks/{taskId}/file/{fileId}, not a big deal, I currently add the requesttoken GET param to display images in the UI.
• Task result files stored in data/appdata_iid/core/TaskProcessing/ are empty for the TextToImage task type (at least). I tried 2 providers (OpenAI and Replicate which are providers for the old TTI API).
• The manager is unable to read input files if they are in a user's storage. I tried an STT provider (for the old STT API), I get this exception raised: https://github.com/nextcloud/server/blob/enh/taskprocessing-api/lib/private/TaskProcessing/Manager.php#L763 which means this does not get the file: https://github.com/nextcloud/server/blob/enh/taskprocessing-api/lib/private/TaskProcessing/Manager.php#L759 . Happy to debug that together if you want.