Can get stuck unable to create or join DM after leaving

[olivia-fl]

Describe the bug

Cinny does not to remove DM rooms from the m.direct global account data event when the rooms are left, causing it to prompt the user to rejoin a left DM room rather than creating a new one. It is not possible to rejoin a room with no joined users, so this can leave you in a state where you can neither create a new DM nor rejoin the old one for a particular user.

Reproduction

1. As @user1, start a DM with @user2
2. Leave the DM room before @user2 joins
3. Attempt to start another DM with @user2 using the UI
4. The previous DM room from step 1 is opened, with a prompt to join. If you try to join, it fails because the room is empty.

Expected behavior

When leaving a DM room, it should be removed from the m.direct map. If none of the rooms in m.direct are joined, cinny should create a new DM (this is needed even if cinny removes left rooms, because another client might not).

Platform and versions

1. OS: Linux (nixos)
2. Browser: librewolf 133.0
3. Cinny version: v4.2.3
4. Matrix homeserver: computer.surgery (running grapevine 8537c0e8ac3eb388500587b035008e5f98204a4b). Also reproduced on envs.net, running synapse.

Additional context

As far as I know, the behavior where it is impossible to join an abandoned room is not documented anywhere, but I asked spec people about this a while ago and was told it is not a bug.