Create notification upon IMip message processing failure

[Juniar-Rakhman]

This is a follow up PR based on feedback on the original PR, which in turn is a follow up changes to a patch done for #10811

The patch to address above issue fixes the processing flow of iMip message processing. When a failure occurs, the batch job logs the error and continues processing the next iMIP message instead of failing the entire job.

Issue:
Although the patch resolves the processing problem, failures occur silently from the user’s perspective. The user is not informed when an iMIP message fails to process and therefore cannot take corrective action.

Solution:
Created a notification upon iMIP messaging failure that will be sent to message owner so they can take corrective actions.

Added the following changes:

1. Error recovery with notification instead of failing silently.
2. Added sendErrorNotification() method to generate Nextcloud notification
3. Added a new notification subject 'imip_processing_failed' handled by the Mail app notifier.
4. When an error occurs, the user receives a Nextcloud notification explaining that the invitation could not be processed automatically.
5. The notification includes a direct link to the affected email, allowing the user to open it and manually add the calendar event.

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[Juniar-Rakhman]

Tested in one of our nextcloud instance and the notification is working well as shown below.

When IMip processing failure occured, a notification is shown to the user.

When clicked, user will be redirected to the failed email message. They can then take corrective action by manually adding the attached invitation to their calendar.

For Users that do not use Nextcloud web, they might missed the notification, but they should receive the email about unread notifications after 1 hour (configurable).

[Juniar-Rakhman]

The changes in this PR are not ideal for us since iMIP messages are time-sensitive and most of our users do not use the Nextcloud web interface.

In our production instance, we address this by implementing both push and email notifications. The email notification sent to the user is similar to the one in the original PR. The custom code changes that have been working for us can be found here.

Ideally, I would prefer to configure this notification similar to the "Activity" notifications:

For example, by adding a setting called "iMIP Processing Failure" under "Other activities".

However, this would likely require more extensive changes to the Notifications app as well.

For now, I think the changes in this PR are sufficient to address the main issue.

[SebastianKrupinski]

Hi @Juniar-Rakhman

Thank you for the PR, please give us some time to review it

[Juniar-Rakhman]

Hi @SebastianKrupinski

Thank you for taking the time to review it. Looking forward to your feedback.

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