fix(imip): set charset for imip attachment

[kesselb]

• Resolves: [Bug]: Charset for imip invitations not set to UTF-8 #53811

Summary

TODO

• Manual testing mail provider
• Manual testing fallback

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)

[kesselb]

cc @ChristophWurst, could you do a smoke test with evolution?

[ChristophWurst]

The invites you have forwarded are processed correctly by Evolution 👍

[kesselb]

Hi,

I wanted a test to ensure we are not losing that requirement for charset=utf-8 in a future change, as it’s a detail that’s easy to miss.

The IMipPlugin sends emails for iTIP messages (e.g. calendar invitations or updates). That involves a couple of dependencies needed to get the job done. It’s therefore no surprise that our unit tests for IMipPlugin make heavy use of test doubles. I often find it hard to find out the actual purpose of a test because the majority of the test code is mocking dependencies.

Therefore, I opted not to add another test to IMipPluginTest but to start from scratch. While it’s currently inevitable to use mocks, I at least want to avoid mocking the “core components” IMipPlugin, IMipService, and EventComparisonService, as they are tightly coupled anyway.

IMipPluginCharsetTest still makes heavy use of test doubles, but it's hopefully a bit easier to understand and has a clearer scope.

[kesselb]

Re-requested a review, because of new test.
I would not backport the test, as it requires PHPUnit 10.

[ChristophWurst]

[kesselb]

/backport to stable31

[kesselb]

/backport to stable30