Skip DXGI device discovery if Win32k system calls are disabled (#29755)
### Description
<!-- Describe your changes. -->
Skip DXGI device discovery when Win32k system calls are disabled.
DXGI internally calls `gdi32!DdQueryDisplaySettingsUniqueness`, which
reads from the GDI shared memory section. Under Win32k lockdown this
section may not be mapped (depending on the host's gdi32 initialization
strategy), causing an access violation.
```
gdi32!DdQueryDisplaySettingsUniqueness+0x7:
00007ff8`30cf45f7 8b8090001800 mov eax,dword ptr [rax+180090h] ds:00000000`00180090=????????
# Child-SP RetAddr Call Site
00 00000072`4b5fc6f8 00007ff8`2b0f583e gdi32!DdQueryDisplaySettingsUniqueness+0x7
01 00000072`4b5fc700 00007ff8`2b0f433f dxgi!CDXGIFactory::SampleAdapters+0xae
02 00000072`4b5fc780 00007ff8`2b0f3f6a dxgi!CDXGIFactory::Initialize+0x10f
03 00000072`4b5fc870 00007ff8`2b0f34b8 dxgi!CreateDXGIFactoryImpl+0x9a
04 00000072`4b5fc8f0 00007fff`a9e08856 dxgi!CreateDXGIFactoryActualImpl2+0x58
05 00000072`4b5fc930 00007fff`a9e075c3 onnxruntime!onnxruntime::`anonymous namespace'::GetDeviceInfoD3D12+0x86 [device_discovery.cc @ 337]
06 00000072`4b5fcc10 00007fff`a9dec8a1 onnxruntime!onnxruntime::DeviceDiscovery::DiscoverDevicesForPlatform+0x323 [device_discovery.cc @ 584]
07 00000072`4b5fd000 00007fff`a9ded12e onnxruntime!`onnxruntime::DeviceDiscovery::GetDevices'::`2'::<lambda_1>::operator()+0x61 [device_discovery_common.cc @ 20]
08 00000072`4b5fd4c0 00007fff`a93b6c46 onnxruntime!onnxruntime::DeviceDiscovery::GetDevices+0x7e [device_discovery_common.cc @ 19]
09 00000072`4b5fd4f0 00007fff`a93b2d7d onnxruntime!onnxruntime::`anonymous namespace'::SortDevicesByType+0x36 [environment.cc @ 781]
```
### Motivation and Context
<!-- - Why is this change required? What problem does it solve?
- If it fixes an open issue, please link to the issue here. -->
WebNN is migrating ORT graph compilation to a sandboxed process with
PROCESS_MITIGATION_SYSTEM_CALL_DISABLE_POLICY.DisallowWin32kSystemCalls
enabled. This prevents calls through GDI for security hardening.
This change has no effect on normal (unsandboxed) processes.