Forcefully leave room on multiplayer exit

[smoogipoo]

Fixes #35886 (comment)

Seems pretty difficult to unit test this so I've only tested via:

diff --git a/osu.Game/Online/Multiplayer/OnlineMultiplayerClient.cs b/osu.Game/Online/Multiplayer/OnlineMultiplayerClient.cs
index 7cfa036dcd..e65d1b0738 100644
--- a/osu.Game/Online/Multiplayer/OnlineMultiplayerClient.cs
+++ b/osu.Game/Online/Multiplayer/OnlineMultiplayerClient.cs
@@ -121,6 +121,8 @@ protected override async Task<MultiplayerRoom> JoinRoomInternal(long roomId, str
 
             Debug.Assert(connection != null);
 
+            await Task.Delay(5000).ConfigureAwait(false);
+
             try
             {
                 return await connection.InvokeAsync<MultiplayerRoom>(nameof(IMultiplayerServer.JoinRoomWithPassword), roomId, password ?? string.Empty).ConfigureAwait(false);

Will document more in comments on the PR itself.

[smoogipoo]

I don't really know why this was added or necessary, because it doesn't look to be. Basically, there's a moment in time (that can actually be quite lengthy due to user lookups), where the room is "joined" but Room hasn't been set. The patch in the OP shows this.

It seems like we should always run through the full leave process?

[bdach]

Originates from 02369ba. Appears it wasn't asked about at review time, probably because it looked obvious / harmless.

[bdach]

seems probably fine

[bdach]

Originates from 02369ba. Appears it wasn't asked about at review time, probably because it looked obvious / harmless.

[bdach]

While we're here doing this, the xmldoc says this can throw:

osu/osu.Game/Online/Multiplayer/IMultiplayerRoomServer.cs

Line 19 in f4049c7

/// <exception cref="NotJoinedRoomException">If the user is not in a room.</exception>

but looking at the server-side implementation I don't see how it can throw because this specific scenario is guarded to do nothing instead of throwing, so might as well strip that mention.