Make conhost act in VtIo mode earlier in startup (#15298)

We need to act like a ConPTY just a little earlier in startup. My relevant notes start here: #15245 (comment). 

Basically, we'd create the first screen buffer with 9001 rows, because it would be created _before_ VtIo would be in a state to say "yes, we're a conpty". Then, if a CLI app emits an entire screenful of text _before_ the terminal has a chance to resize the conpty, then the conpty will explode during `_DoLineFeed`. That method is absolutely not expecting the buffer to get resized (and the old text buffer deallocated). 

Instead, this will treat the console as in ConPty mode as soon as `VtIo::Initialize` is called (this is during `ConsoleCreateIoThread`, which is right at the end of `ConsoleEstablishHandoff`, which is before the API server starts to process the client connect message).  THEORETICALLY, `VtIo` could `Initialize` then fail to create objects in `CreateIoHandlers` (which is what we used to treat as the moment that we were in conpty mode). However, if we do fail out of `CreateIoHandlers`, then the console itself will fail to start up, and just die. So I don't think that's needed.

This fixes #15245. I think this is PROBABLY also the solution to #14512, but I'm not gonna explicitly mark closed. We'll loop back on it.

src/host/VtIo.cpp

@@ -25,7 +25,6 @@ using namespace Microsoft::Console::Interactivity;
 
 VtIo::VtIo() :
     _initialized(false),
-    _objectsCreated(false),
     _lookingForCursorPosition(false),
     _IoMode(VtIoMode::INVALID)
 {
@@ -224,13 +223,12 @@ VtIo::VtIo() :
     }
     CATCH_RETURN();
 
-    _objectsCreated = true;
     return S_OK;
 }
 
 bool VtIo::IsUsingVt() const
 {
-    return _objectsCreated;
+    return _initialized;
 }
 
 // Routine Description:
@@ -246,7 +244,7 @@ bool VtIo::IsUsingVt() const
 [[nodiscard]] HRESULT VtIo::StartIfNeeded()
 {
     // If we haven't been set up, do nothing (because there's nothing to start)
-    if (!_objectsCreated)
+    if (!_initialized)
     {
         return S_FALSE;
     }
@@ -514,7 +512,7 @@ void VtIo::EndResize()
 // - <none>
 void VtIo::EnableConptyModeForTests(std::unique_ptr<Microsoft::Console::Render::VtEngine> vtRenderEngine)
 {
-    _objectsCreated = true;
+    _initialized = true;
     _pVtRenderEngine = std::move(vtRenderEngine);
 }
 #endif

src/host/VtIo.hpp

@@ -63,7 +63,6 @@ namespace Microsoft::Console::VirtualTerminal
         VtIoMode _IoMode;
 
         bool _initialized;
-        bool _objectsCreated;
 
         bool _lookingForCursorPosition;