Initialize *ppv in comhost DllGetClassObject (#128916)

`DllGetClassObject` in `src/native/corehost/comhost/comhost.cpp` had
failure paths (CLSID not in map, `get_com_delegate` failure) that
returned a failure HRESULT without writing to `*ppv`, violating the COM
contract that out parameters must always be initialized.

Null it out on function entry.

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: elinor-fung <47805090+elinor-fung@users.noreply.github.com>

Author: Copilot

src/native/corehost/comhost/comhost.cpp

@@ -198,6 +198,12 @@ COM_API HRESULT STDMETHODCALLTYPE DllGetClassObject(
     _In_ REFIID riid,
     _Outptr_ LPVOID FAR* ppv)
 {
+    if (ppv == nullptr)
+        return E_POINTER;
+
+    // COM out parameters must always be initialized, including on failure paths.
+    *ppv = nullptr;
+
     // Check if the CLSID map contains a mapping
     clsid_map map;
     RETURN_HRESULT_IF_EXCEPT(map = comhost::get_clsid_map());