Avoid reentrancy issues when dropping AppHost, even harder (#19395)

The previous fix in #19296 moved the _destruction_ of AppHost into the
tail end after we manipulate the `_windows` vector; however, it kept the
part which calls into XAML (`Close`) before the `erase`. I suspect that
we still had some reentrancy issues, where we cached an iterator before
the list was modified by another window close event.

That is:

```mermaid
sequenceDiagram
		Emperor->>Emperor: Close Window
		Emperor->>+AppHost: Close (a)
		AppHost->>XAML: Close
		XAML-->>Emperor: pump loop
		Emperor->>Emperor: Close Window
		Emperor->>+AppHost: Close (b)
		AppHost->>XAML: Close
		XAML-->>Emperor: pump loop
		AppHost->>-Emperor: Closed
		Emperor->>Emperor: erase(b)
		AppHost->>-Emperor: Closed
		Emperor->>Emperor: erase(a)
```

Moving the `Close()` to after the `erase` ensures that there are no
cached iterators that survive beyond XAML pumping the message loop.

Fixes 8d41ace

(cherry picked from commit 5976de1)
Service-Card-Id: PVTI_lADOAF3p4s4BBcTlzgfScpM
Service-Version: 1.24

Author: DHowett

src/cascadia/WindowsTerminal/WindowEmperor.cpp

@@ -880,16 +880,16 @@ LRESULT WindowEmperor::_messageHandler(HWND window, UINT const message, WPARAM c
                 {
                     if (host == it->get())
                     {
-                        // NOTE: The AppHost destructor is highly non-trivial.
+                        // NOTE: AppHost::Close is highly non-trivial.
                         //
                         // It _may_ call into XAML, which _may_ pump the message loop, which would then recursively
                         // re-enter this function, which _may_ then handle another WM_CLOSE_TERMINAL_WINDOW,
                         // which would change the _windows array, and invalidate our iterator and crash.
                         //
-                        // We can prevent this by deferring destruction until after the erase() call.
+                        // We can prevent this by deferring Close() until after the erase() call.
                         const auto strong = *it;
-                        strong->Close();
                         _windows.erase(it);
+                        strong->Close();
                         break;
                     }
                 }