fix(iMIP): Prevent mails from carrying an unrelated user's name

[DerDreschner]

Summary

This PR fixes an issue in the iMIP component of Nextcloud.

When the iTip message contains no (optional) CN property for the person we're sending the iMIP message on behalf of, we're using the display name of the current session user to fill out the sender for iMIP messages. This means we're sending out mails with the correct mail address, but wrong display name. Example how it's sent out right now, although it should say "A":

From: "B via Nextcloud" <noreply@...>
Reply-To: "B" <a@test.example>

This is especially the case when using Thunderbird, as it doesn't set the CN property.

To solve the issue, this PR implements the following steps:

1. Does the mail address matches with one of the current session user's addresses? If so, we're still using the display name of the current session.
2. If it doesn't, we're looking up if we know exactly one user with the provided mail address and use their display name
3. If we don't know any user with that mail address, we're falling back to a neutral From: "Nextcloud" <noreply...>; Reply-To: <a@test.example>

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

[DerDreschner]

/backport to stable34

[DerDreschner]

/backport to stable33

[DerDreschner]

/backport to stable32

[SebastianKrupinski]

Hey,

I had a quick look and read and I think we are going to have to make this change further down in the schedule() function, there are two sending code paths, one for sending via system mail the other using mail provider (user own email address)

When sending using the mail provider we use the email address that is original provided in the event to match a email account in the mail provider.

So it should be:

System mail -> Set display name -> Set reply to address
Mail Provider -> use address to match account