fix(ground-suite): handle failed UTF-16 conversion sizes

Reject failed UTF-16 sizing queries before allocating the UTF-8 result
and preserve terminating-NUL accounting.

Validation: nine host-stub cases with 16-bit wchar_t; native Windows
execution remains unverified.

ground_suite/windows/runner/utils.cpp

@@ -45,21 +45,24 @@ std::string Utf8FromUtf16(const wchar_t* utf16_string) {
   if (utf16_string == nullptr) {
     return std::string();
   }
-  unsigned int target_length = ::WideCharToMultiByte(
+  const int target_length = ::WideCharToMultiByte(
       CP_UTF8, WC_ERR_INVALID_CHARS, utf16_string,
-      -1, nullptr, 0, nullptr, nullptr)
-    -1; // remove the trailing null character
-  int input_length = (int)wcslen(utf16_string);
+      -1, nullptr, 0, nullptr, nullptr);
+  if (target_length <= 0) {
+    return std::string();
+  }
   std::string utf8_string;
-  if (target_length == 0 || target_length > utf8_string.max_size()) {
+  if (static_cast<size_t>(target_length) > utf8_string.max_size()) {
     return utf8_string;
   }
+  // With a terminated input, both calls include the trailing null character.
   utf8_string.resize(target_length);
   int converted_length = ::WideCharToMultiByte(
       CP_UTF8, WC_ERR_INVALID_CHARS, utf16_string,
-      input_length, utf8_string.data(), target_length, nullptr, nullptr);
-  if (converted_length == 0) {
+      -1, utf8_string.data(), target_length, nullptr, nullptr);
+  if (converted_length != target_length) {
     return std::string();
   }
+  utf8_string.resize(static_cast<size_t>(converted_length) - 1);
   return utf8_string;
 }