fix(host): break InputBar/CommandHelpWindow/MainWindow DI cycle

J + J2 closed a singleton cycle:
  InputBar.ctor -> CommandHelpWindow (J2)
  CommandHelpWindow.ctor -> MainWindow (J)
  MainWindow.ctor -> InputBar (pre-existing)

MS.DI does not detect cycles through FactoryCallSite registrations,
so resolution recursed silently on the async plugin-init thread until
the worker died with an uncatchable StackOverflowException. Dalamud's
LoadAsync task never resolved; the plugin UI hung on "Enabling..."
with no exception in the log. First triggered at Plugin.cs:289
(TypingIpc.ctor needs InputBar).

Fix: break the cycle on the laziest edge.
  - CommandHelpWindow.ctor no longer takes MainWindow.
  - New AttachMainWindow setter wired in
    CommandHelpWindowInitHostedService.StartAsync, mirroring the
    existing §6.2 MessageList.AttachPayloadHandler pattern.
  - UpdateContent throws InvalidOperationException if the setter
    never ran, so a future regression fails loudly instead of a
    silent NullRef during input draw.

Also enable UseDefaultServiceProvider(ValidateOnBuild + ValidateScopes)
so future ConstructorCallSite cycles throw at Build time instead of
silently hanging. Catches reflection-based registrations; will not
catch FactoryCallSite cycles like this one (those still need code review).

Verified via 6 enable/disable cycles in-game; plugin loads cleanly,
Hosting starts, FilterAllTabs completes, command help popup renders
for /em and /say (exercises AttachMainWindow), hover counter ticks
(exercises PayloadHandlerInitHostedService AddonLifecycle wiring).

HellionChat/Ui/CommandHelpWindow.cs

@@ -13,20 +13,21 @@ namespace HellionChat.Ui;
 internal sealed class CommandHelpWindow : Window
 {
     private readonly ChunkRenderer _chunkRenderer;
-    private readonly Windows.MainWindow _mainWindow;
     private readonly ILogger<CommandHelpWindow> _logger;
 
+    // Setter-injected post-ctor to break the InputBar -> CommandHelpWindow ->
+    // MainWindow -> InputBar singleton cycle (MS.DI does not detect cycles
+    // through FactoryCallSite registrations). Wired in
+    // CommandHelpWindowInitHostedService.StartAsync, same §6.2 pattern as
+    // MessageList.AttachPayloadHandler.
+    private Windows.MainWindow? _mainWindow;
+
     private ReadOnlySeString? _commandDescription;
 
-    internal CommandHelpWindow(
-        ChunkRenderer chunkRenderer,
-        Windows.MainWindow mainWindow,
-        ILogger<CommandHelpWindow> logger
-    )
+    internal CommandHelpWindow(ChunkRenderer chunkRenderer, ILogger<CommandHelpWindow> logger)
         : base("command help##chat2-commandhelp")
     {
         _chunkRenderer = chunkRenderer;
-        _mainWindow = mainWindow;
         _logger = logger;
 
         Flags =
@@ -44,8 +45,18 @@ internal sealed class CommandHelpWindow : Window
         _ = _logger;
     }
 
+    internal void AttachMainWindow(Windows.MainWindow mainWindow) => _mainWindow = mainWindow;
+
     public void UpdateContent(ReadOnlySeString commandDesc)
     {
+        // Loud-fail if the HostedService didn't run AttachMainWindow before
+        // the first slash-command call — better than a silent NullRef during
+        // input draw.
+        if (_mainWindow is null)
+            throw new InvalidOperationException(
+                "CommandHelpWindow.UpdateContent called before AttachMainWindow."
+            );
+
         _commandDescription = commandDesc;
 
         var width = 350;