test(selftests): add FontManager ctor and push smoke steps

Two new self-test steps for the hybrid FontManager:

- FontManagerCtorSmokeStep proves all five handles land on the manager
  after Phase-1 resolve (ItalicFont nullable per Config.ItalicEnabled)
  and that no atlas-load exception is sitting on any of them
- FontPushSmokeStep proves IFontHandle.Push() returns without throwing
  for the two main delegate handles right after plugin load

Both steps run on the framework thread via the xlperf self-test path
and are registered alongside the existing theme-switch step in
SelfTestRegistry.

HellionChat/Plugin.cs

@@ -317,7 +317,11 @@ public sealed class Plugin : IAsyncDalamudPlugin
             if (_lifecycle is not null)
                 await _lifecycle.LoadAsync(cancellationToken).ConfigureAwait(false);
 
-            SelfTestRegistry.RegisterTestSteps([new SelfTests.ThemeSwitchSelfTestStep(this)]);
+            SelfTestRegistry.RegisterTestSteps([
+                new SelfTests.ThemeSwitchSelfTestStep(this),
+                new SelfTests.FontManagerCtorSmokeStep(this),
+                new SelfTests.FontPushSmokeStep(this),
+            ]);
 
             if (!Config.FirstRunCompleted)
                 FirstRunWizard.IsOpen = true;