BUG:[windows] Missing error handling for RegisterClass in win32_window.cpp

### Is there an existing issue for this?

- [x] I have searched the existing issues

### What happened?

**Issue Overview**
Some critical WinAPI calls such as RegisterClass and DwmSetWindowAttribute are used without checking their return values. This can make debugging difficult if these calls fail silently.

**Steps to Reproduce**
Inspect the implementation of WindowClassRegistrar::GetWindowClass()
Observe that RegisterClass is called without checking its return value
Inspect Win32Window::UpdateTheme()
Observe that DwmSetWindowAttribute is called without verifying success

**Expected Behavior**
WinAPI calls should have their return values checked
Failures should be logged or handled appropriately

**Actual Behavior**
Failures in these API calls are not detected
Debugging issues related to window creation or theme updates becomes difficult

**Suggested Improvements**
Add minimal error handling for these calls:
if (!RegisterClass(&window_class)) {
OutputDebugString(L"Failed to register window class\n");
}
HRESULT hr = DwmSetWindowAttribute(...);
if (FAILED(hr)) {
OutputDebugString(L"Failed to set dark mode attribute\n");
}

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