refactor(messagestore): inject IPluginLogProxy for test isolation

MessageStore's Migrate0 (and the Migrate1/2/3 siblings) called
Plugin.Log.Information directly, which prevented an isolated xUnit
construction test from running — Dalamud.dll cannot load in the test
AppDomain. With IPluginLogProxy threaded through the ctor and the inner
MessageEnumerator, the whole MessageStore.cs file is now Dalamud-static
free and the Build-Suite covers it (Floor 688 -> 690).

This is the second half of F12.2; the remaining ~82 Plugin.Log call
sites in the rest of the plugin will be routed through the static
Plugin.LogProxy wrapper in a follow-up commit.

HellionChat/MessageManager.cs

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