As seen by this kind of crash, having the `.ToMod` method throw can be
very problematic and also hidden (as it is used inside of models in
places where exceptions are not expected to occur).
Given there are tens of usages of this method, returning a placeholder
mod seems like a better idea than outright throwing.
```
An unhandled has occurred.
System.InvalidOperationException:
There is no mod in the ruleset (osu) matching the acronym AS.
at osu.Game.Online.API.APIMod.ToMod(Ruleset ruleset) in /Users/dean/Projects/osu/osu.Game/Online/API/APIMod.cs:line 54
at osu.Game.Scoring.ScoreInfo.<get_Mods>b__117_0(APIMod m) in /Users/dean/Projects/osu/osu.Game/Scoring/ScoreInfo.cs:line 193
at System.Linq.Enumerable.SelectArrayIterator`2.ToArray()
at osu.Game.Scoring.ScoreInfo.get_Mods() in /Users/dean/Projects/osu/osu.Game/Scoring/ScoreInfo.cs:line 193
at osu.Game.Screens.Select.Leaderboards.BeatmapLeaderboard.<>c.<subscribeToLocalScores>b__40_2(ScoreInfo s) in /Users/dean/Projects/osu/osu.Game/Screens/Select/Leaderboards/BeatmapLeaderboard.cs:line 199
at System.Linq.Enumerable.WhereEnumerableIterator`1.MoveNext()
at osu.Game.Database.RealmObjectExtensions.Detach[T](IEnumerable`1 items) in /Users/dean/Projects/osu/osu.Game/Database/RealmObjectExtensions.cs:line 180
at osu.Game.Screens.Select.Leaderboards.BeatmapLeaderboard.<>c__DisplayClass40_0.<subscribeToLocalScores>g__localScoresChanged|1(IRealmCollection`1 sender, ChangeSet changes, Exception exception) in /Users/dean/Projects/osu/osu.Game/Screens/Select/Leaderboards/BeatmapLeaderboard.cs:line
209
at Realms.RealmCollectionBase`1.Realms.INotifiable<Realms.NotifiableObjectHandleBase.CollectionChangeSet>.NotifyCallbacks(Nullable`1 changes, Nullable`1 exception)
at Realms.NotifiableObjectHandleBase.NotifyObjectChanged(IntPtr managedHandle, IntPtr changes, IntPtr exception)
```
This wasn't being used by osu-web, and included far too much unnecessary
data. Of note, `pp` and `ruleset_id` are also not strictly required, but
there's no harm in sending them so I've left them be for now.
This can happen if the user leaves the channel before the request is
fired. You can't mark a channel as read when you're not in the channel.
Addresses https://github.com/ppy/osu/discussions/16973.
I remember that this conditional copy was added to support making copies
of spotlight rooms without carrying across the `Spotlight` type, but in
testing this is already handled web side to the point that it's not
required.
The rationale for allowing the copy is that this method is used for
tests, where it was not being copied correctly from the input as
expected (used at
bdc3b76df0/osu.Game/Tests/Visual/OnlinePlay/TestRoomManager.cs (L38)).
