PHP fatal error when saving tasks with attachments

Problem/Motivation

Since https://www.drupal.org/project/minikanban/issues/3529811 was merged, if a task has more than 1 attachment a php fatal error occurs when saving the task.

PHP Fatal error:  Nesting level too deep - recursive dependency? in /var/www/html/docroot/modules/contrib/minikanban/src/Entity/Task.php on line 166

Steps to reproduce

// Load files that already exist.
$file = \Drupal\file\Entity\File::load(1);
$file2 = \Drupal\file\Entity\File::load(2);

// Create a new task.
$mytask = \Drupal\minikanban\Entity\Task::create();

// Add the file to the task and save.
$mytask->get('attachments')->appendItem($file);
$mytask->get('attachments')->appendItem($file2);

// Should save, but generates a php fatal.
$mytask->save();

Proposed resolution

The above is caused due to the array_unique call in Task::preSave (line 166), as array_unique converts objects to strings to do the comparison.

Doing a uniqueness check on the file IDs rather than the file entities themselves should resolve this, but this would require ensuring that the files have already been saved before adding them to the attachments collection and I'm unsure if this is a safe assumption.

Remaining tasks

User interface changes

API changes

Data model changes