fix: cannot edit calendar event with external attendees (#8050)

[chandrika1993]

fixes: #8050

@GretaD
When an event has external attendees (e.g. user@gmail.com), the CalDAV server rejects the save because it tries to resolve those email addresses as local users and fails.

The fix adds SCHEDULE-AGENT=CLIENT to external attendee properties so the server knows not to handle scheduling for addresses it cannot resolve locally (RFC 6638 Section 7.1).

Also fixed the calendar grid not updating instantly after saving. The server does not return an ETag on 204 responses, which caused an unnecessary extra request before the UI could refresh. A background PROPFIND now restores the ETag immediately after saving, so the grid updates instantly.

[chandrika1993]

The build failure looks like a JavaScript heap out of memory error on the CI runner rather than an issue with the code changes. Could a maintainer take a look and re-run the failing jobs if needed? Happy to investigate further if it turns out to be related to the PR.

[SebastianKrupinski]

Hi @chandrika1993

Thank you for the PR. I will look through it soon.

[GVodyanov]

Tested the PR and it doesn't seem to break anything... but I also wasn't able to reproduce the original issue so I guess that doesn't mean too much

[tcitworld]

This looks wrong. The server should send invitations regardless if e-mails are matching local users or not. The fix should be on the server's side.

[chandrika1993]

This looks wrong. The server should send invitations regardless if e-mails are matching local users or not. The fix should be on the server's side.

Thanks for the feedback! Agreed that a server-side fix would be the right long-term solution.

The reason for the client-side workaround is that the server is currently rejecting the entire PUT request when it can't resolve an external attendee, so users are completely blocked from editing those events. SCHEDULE-AGENT=CLIENT is intended as a temporary fix to unblock users until the server behaviour is corrected.

Happy to hear if there is a better approach!