Fix handling of "%s" in vararg functions in some UTF-8 builds

When wxUSE_UNICODE_UTF8 is set to 1, strings use UTF-8 and are still
char strings and not wchar_t ones, but when using wx own implementation
of vsnprintf(), "%s" expected a wide string and not a normal one.

Fix this at wxPrintfFormatConverterUtf8 level.

Closes #26236.

src/common/strvararg.cpp

@@ -478,7 +478,14 @@ class wxPrintfFormatConverterUtf8 : public wxFormatConverterBase<char>
                               CharType& outConv, SizeModifier& outSize) override
     {
         outConv = 's';
+#if wxUSE_WXVSNPRINTFA
+        // When using wx's own vsnprintf implementation, plain %s is
+        // interpreted as wchar_t*. We need %hs to indicate char* args,
+        // which is what we actually pass in UTF-8 builds.
+        outSize = Size_Short;
+#else
         outSize = Size_Default;
+#endif
     }
 
     virtual void HandleChar(CharType WXUNUSED(conv),