A redacted message at the end of a thread is never read

[andybalaam]

• As user A, look at some other room.
• As user B, create a thread in a room with several messages in it.
• As user B, redact the last message in the thread.
• As user A, open the room and the thread.

User A sees the room as unread in the room list, even though they have read all messages.

I would expect the room to be read.

Testcase: "Reading an unread thread after a redaction of the latest message makes it read" in cypress/e2e/read-receipts/redactions.spec.ts.

Part of #24392

[andybalaam]

Related: MSC3389: Relation redactions which proposes to preserve relation information through redactions.

[andybalaam]

The problem here is that to be consistent with the spec (and the server) we need to move messages out of the thread when they are redacted. I will work on a change to do that.