[SHELL32][SHELL32_APITEST][SDK] Implement SHGetUserDisplayName (#7612)

Implemementing missing features...
JIRA issue: CORE-19278
- Add netapi32 and secur32 delay importing.
- Move function definition from stubs.cpp into utils.cpp.
- Include some security headers in utils.cpp.
- Adapt <secext.h> to C++.
- Add prototype to <undocshell.h>.

modules/rostests/apitests/shell32/SHGetUserDisplayName.cpp

@@ -0,0 +1,30 @@
+/*
+ * PROJECT:     ReactOS API tests
+ * LICENSE:     GPL-2.0+ (https://spdx.org/licenses/GPL-2.0+)
+ * PURPOSE:     Test for SHGetUserDisplayName
+ * COPYRIGHT:   Copyright 2025 Katayama Hirofumi MZ (katayama.hirofumi.mz@gmail.com)
+ */
+
+#include "shelltest.h"
+#include <undocshell.h>
+
+START_TEST(SHGetUserDisplayName)
+{
+    HRESULT hr;
+    WCHAR szBuf[MAX_PATH];
+    ULONG cchBuf;
+
+    hr = SHGetUserDisplayName(NULL, NULL);
+    ok_hex(hr, E_INVALIDARG);
+
+    hr = SHGetUserDisplayName(szBuf, NULL);
+    ok_hex(hr, E_INVALIDARG);
+
+    cchBuf = _countof(szBuf);
+    hr = SHGetUserDisplayName(NULL, &cchBuf);
+    ok_hex(hr, E_INVALIDARG);
+
+    cchBuf = _countof(szBuf);
+    hr = SHGetUserDisplayName(szBuf, &cchBuf);
+    ok_hex(hr, S_OK);
+}