Eagerly persist on WM_ENDSESSION (#18912)

Once an application has returned from handling `WM_ENDSESSION`
the OS may kill the app at any point. This means we must
persist our state while inside the message handler.

Closes #17179

## Validation Steps Performed

Honestly, none. It's a race condition in the first place.

src/cascadia/WindowsTerminal/WindowEmperor.cpp

@@ -899,7 +899,8 @@ LRESULT WindowEmperor::_messageHandler(HWND window, UINT const message, WPARAM c
             RegisterApplicationRestart(nullptr, RESTART_NO_CRASH | RESTART_NO_HANG);
             return TRUE;
         case WM_ENDSESSION:
-            _forcePersistence = true;
+            _finalizeSessionPersistence();
+            _skipPersistence = true;
             PostQuitMessage(0);
             return 0;
         default:
@@ -946,7 +947,7 @@ void WindowEmperor::_persistState(const ApplicationState& state, bool serializeB
         state.PersistedWindowLayouts(nullptr);
     }
 
-    if (_forcePersistence || _app.Logic().Settings().GlobalSettings().ShouldUsePersistedLayout())
+    if (_app.Logic().Settings().GlobalSettings().ShouldUsePersistedLayout())
     {
         for (const auto& w : _windows)
         {
@@ -960,6 +961,13 @@ void WindowEmperor::_persistState(const ApplicationState& state, bool serializeB
 
 void WindowEmperor::_finalizeSessionPersistence() const
 {
+    if (_skipPersistence)
+    {
+        // We received WM_ENDSESSION and persisted the state.
+        // We don't need to persist it again.
+        return;
+    }
+
     const auto state = ApplicationState::SharedInstance();
 
     _persistState(state, true);

src/cascadia/WindowsTerminal/WindowEmperor.h

@@ -77,7 +77,7 @@ private:
     UINT WM_TASKBARCREATED = 0;
     HMENU _currentWindowMenu = nullptr;
     bool _notificationIconShown = false;
-    bool _forcePersistence = false;
+    bool _skipPersistence = false;
     bool _needsPersistenceCleanup = false;
     SafeDispatcherTimer _persistStateTimer;
     std::optional<bool> _currentSystemThemeIsDark;