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/Infrastructure/Hosting/InitHostedServices.cs

@@ -3,7 +3,9 @@ using Dalamud.Plugin;
 using HellionChat.Integrations;
 using HellionChat.Ipc;
 using HellionChat.Themes;
+using HellionChat.Ui;
 using HellionChat.Ui.Components;
+using HellionChat.Ui.Windows;
 using Microsoft.Extensions.Hosting;
 
 namespace HellionChat.Infrastructure.Hosting;
@@ -142,3 +144,23 @@ internal sealed class PayloadHandlerInitHostedService(
         });
     }
 }
+
+// Wires MainWindow into CommandHelpWindow post-container-build. CommandHelpWindow
+// cannot take MainWindow as a ctor-param because that would close the cycle
+// InputBar -> CommandHelpWindow -> MainWindow -> InputBar (MS.DI does not catch
+// it through FactoryCallSite registrations and the resolve recurses silently).
+// Both singletons exist by host.StartAsync time, so this is the first safe point
+// to wire the setter — same §6.2 pattern as MessageList.AttachPayloadHandler.
+internal sealed class CommandHelpWindowInitHostedService(
+    CommandHelpWindow commandHelpWindow,
+    MainWindow mainWindow
+) : IHostedService
+{
+    public Task StartAsync(CancellationToken cancellationToken)
+    {
+        commandHelpWindow.AttachMainWindow(mainWindow);
+        return Task.CompletedTask;
+    }
+
+    public Task StopAsync(CancellationToken cancellationToken) => Task.CompletedTask;
+}