fix(dav): keep a cancelled occurrence cancelled for attendees

[ndo84bw]

• Resolves: Recurrent Events: When the organizer changes the status of an occurrence to 'cancelled', the whole event becomes cancelled on the attendee's calendar calendar#6655 (together with fix(CalDAV): iTipBroker message generation and testing #50843)

Summary

Follow-up to and complement of #50843. While testing that iTip broker rework I noticed that cancelling a single occurrence of a recurring event still does not behave correctly for attendees, so this small addition makes the two changes together fully resolve nextcloud/calendar#6655.

Background. nextcloud/calendar#6655 was reported because cancelling a single occurrence cancelled the whole series on the attendee's calendar. That over-cancellation is already fixed by #50843 - the reworked broker no longer turns a single-occurrence cancellation into a series-wide CANCEL. (#50843 is merged but, as far as I can tell, not yet released, which is probably why the issue still looks open.)

The remaining issue. With #50843's broker the cancelled occurrence no longer takes down the whole series, but it does not stay cancelled for attendees either: it reappears as a normal event. TipBroker::parseEventForOrganizer emits a per-instance CANCEL for the cancelled occurrence plus a REQUEST for the attendee's remaining instances, with the cancelled instance removed from the REQUEST. But Sabre\VObject\ITip\Broker::processMessageRequest replaces all components of the attendee's stored object with the ones from the message, so the master-only REQUEST (delivered right after the CANCEL) drops the STATUS:CANCELLED override the CANCEL had just added, and the recurrence regenerates the occurrence as a normal event.

Fix. Keep the cancelled instance in the REQUEST. Because processMessageRequest does a full component replace, the REQUEST has to carry the complete state of the attendee's object - including the cancelled override - otherwise it is dropped. The separate per-instance CANCEL is left in place.

Testing. Verified on a live instance (#50843's broker + this patch): cancelling a single occurrence leaves only that occurrence struck through on both the organizer's and the attendee's calendars; the rest of the series is untouched. Without the patch the occurrence reappeared as a normal event for the attendee. A unit test (testParseEventForOrganizerCreatedInstanceCancelled) was extended to assert the REQUEST includes the cancelled override.

Note. #50843 is on stable34/master only, so this targets master; a stable34 backport pairs with #50843, and on NC 33 the whole-series cancellation persists until #50843 + this change are backported to stable33.

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• Tests (unit, integration, api and/or acceptance) are included
• Screenshots before/after for front-end changes
• Documentation (manuals or wiki) has been updated or is not required
• Backports requested where applicable (ex: critical bugfixes)
• Labels added where applicable (ex: bug/enhancement, 3. to review, feature component)
• Milestone added for target branch/version (ex: 32.x for stable32)

AI (if applicable)

• The content of this PR was partly or fully generated using AI

[ChristophWurst]

Thanks for the contribution

Mind restoring the pull request template and filling it out? Thanks

[ndo84bw]

Mind restoring the pull request template and filling it out? Thanks

🤦‍♂️ Restored and filled out the template, thanks!

[SebastianKrupinski]

Hi @ndo84bw

Thanks for the PR, I will test this when I have a min