refactor(store): route MessageStore IsWine probe through IPlatformUtil (F12.1)

MessageStore.Connect used to call Util.IsWine() directly via a
DalamudUtil alias, which made the ctor unreachable from the xUnit
test AppDomain: any test that allocated a MessageStore tripped a
FileNotFoundException on Dalamud.dll before reaching the assertion.

The ctor now takes an IPlatformUtil and reads the cached IsWine
property. MessageManager passes Plugin.PlatformUtil in. Production
behaviour is identical; the test path can now substitute a fake
and exercise the SQLite migration logic in isolation.

HellionChat/MessageManager.cs

@@ -52,7 +52,7 @@ internal class MessageManager : IAsyncDisposable
     {
         Plugin = plugin;
 
-        Store = new MessageStore(DatabasePath());
+        Store = new MessageStore(DatabasePath(), Plugin.PlatformUtil);
 
         PendingMessageThread = new Thread(() =>
             ProcessPendingMessages(PendingThreadCancellationToken.Token)