Thread summary: outdated/wrong latest event is displayed

When enabling the threads feature in Element X, there were several reports of thread summaries not accurately displaying the latest event.

I'm adding the observed weird behaviours here so we can keep track of them, maybe they can be split into separate issues once we know if they're caused by the same or different causes.

- Sometimes an UTD event will be displayed, even if the event is decrypted and is displayed fine when opening the thread timeline.
- Sometimes an 'unsupported event' message will be displayed instead of the actual event - @bnjbvr mentioned this could be caused by edited events if I'm not mistaken.
- Editing messages in the thread is not reflected in the latest event either: if you edit the latest event in the thread, the previous content will still be visible.
- And I was able to see a previous event than the actual latest one being displayed as the latest one instead, even when the bundled latest event in the event JSON had the right info - maybe a cache issue?

No files, tests, or entry points are named. Start by reproducing the thread-summary cases in Element X and compare the summary with the bundled latest event and the opened thread timeline. Done means the latest summary consistently shows the correct decrypted, edited, supported, and newest event.

Written by the indexing model from the issue text.