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

When the organizer cancels a single occurrence of a recurring event, the attendee receives a "Cancelled" email, but the occurrence still shows up as not cancelled in the attendee's calendar. Both users are on the same Nextcloud instance. (tested on master)

The TipBroker first sends the CANCEL and then follows up with a REQUEST that does not include the cancelled instance. This overwrites the cancellation for that instance: Sabre's processMessageRequest replaces the attendee's entire stored object with the components of the REQUEST.

The patch:

1. TipBroker: keeps the cancelled instances in the REQUEST
2. IMipPlugin: still sends the Cancelled email as before, but filters the invitation email for newly cancelled occurrences (only those come with a CANCEL email).
3. TipBroker: overrides parseEventForAttendee to take the STATUS from the master instead of the last VEVENT. Otherwise, the whole event counts as cancelled and the attendee's REPLYs no longer reach the organizer.

Note:

• A STATUS:CANCELLED override inside a REQUEST is not foreseen by RFC 5546 3.2.2 (STATUS there: TENTATIVE/CONFIRMED); external clients still receive the unchanged, RFC-conformant per-instance CANCEL as the authoritative signal.
• Backport only paired with the broker rework fix(CalDAV): iTipBroker message generation and testing #50843, which exists as of stable34 only.

Tested:

• Unit: 47 tests green (16 IMipPluginTest, 31 TipBrokerTest), 5 of them new: REQUEST email suppression for a pure cancellation; the email is kept when cancelling a pre-existing override; a real change with a cancelled override riding along mails the real change; attendee REPLY despite a cancelled override in the copy; the REQUEST keeps the cancelled instance.
• Scripted (CalDAV/curl, AI-assisted): after the cancellation, the attendee's stored copy keeps the cancelled override (without the patch: only the master remains); email counts: keeping the override without the IMipPlugin filter produced an additional bogus "Invitation:" email per cancellation, with the filter only the Cancelled email goes out.
• Manual (web UI + Thunderbird, without accepting beforehand): the occurrence stays struck through in the attendee's calendar instead of reappearing as a normal event; only the Cancelled email arrives; accepting/declining via Thunderbird after the cancellation reaches the organizer (email + participation status); moving an occurrence and then cancelling it still produces an update email.

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
• analyze the code and find the root cause
• Suggest Code
• Wrote and ran Unit Tests
• Translation of PR Body

[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

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