win: symbol exports and minor fixes (#3024)

Co-authored-by: Cheng <[email protected]>

tests/tests.cpp

@@ -15,7 +15,9 @@ int main(int argc, char** argv) {
   const char* device = std::getenv("DEVICE");
   if (device != nullptr && std::string(device) == "cpu") {
     set_default_device(Device::cpu);
-  } else if (metal::is_available()) {
+  } else if (is_available(Device::gpu)) {
+    // Use generic GPU availability check (works for Metal on macOS, or CUDA on
+    // Linux/Windows)
     set_default_device(Device::gpu);
   }