take advantage of SpanMarshalling for ADsGetLastError

suggested by review comment #93722 (comment)

src/libraries/Common/src/Interop/Windows/Activeds/Interop.ADsGetLastError.cs

@@ -1,13 +1,14 @@
 // Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
 
+using System;
 using System.Runtime.InteropServices;
 
 internal static partial class Interop
 {
     internal static partial class Activeds
     {
-        [LibraryImport(Libraries.Activeds, StringMarshalling = StringMarshalling.Utf16)]
-        internal static unsafe partial int ADsGetLastError(out int error, char* errorBuffer, int errorBufferLength, char* nameBuffer, int nameBufferLength);
+        [LibraryImport(Libraries.Activeds)]
+        internal static partial int ADsGetLastError(out int error, Span<char> errorBuffer, int errorBufferLength, Span<char> nameBuffer, int nameBufferLength);
     }
 }

src/libraries/System.DirectoryServices/src/System/DirectoryServices/DirectoryServicesCOMException.cs

@@ -53,14 +53,15 @@ internal static Exception CreateFormattedComException(int hr)
             return CreateFormattedComException(new COMException(errorMsg, hr));
         }
 
-        internal static unsafe Exception CreateFormattedComException(COMException e)
+        internal static Exception CreateFormattedComException(COMException e)
         {
             // get extended error information
             const int ErrorBufferLength = 256;
-            char* errorBuffer = stackalloc char[ErrorBufferLength];
-            char nameBuffer = '\0';
+            Span<char> errorBuffer = stackalloc char[ErrorBufferLength];
+
+            Span<char> nameBuffer = Span<char>.Empty;
             int error = 0;
-            Interop.Activeds.ADsGetLastError(out error, errorBuffer, ErrorBufferLength, &nameBuffer, 0);
+            Interop.Activeds.ADsGetLastError(out error, errorBuffer, errorBuffer.Length, nameBuffer, nameBuffer.Length);
 
             if (error != 0)
                 return new DirectoryServicesCOMException(new string(errorBuffer), error, e);

src/libraries/System.DirectoryServices/src/System/DirectoryServices/SearchResultCollection.cs

@@ -401,11 +401,12 @@ private void CleanLastError()
                 Interop.Activeds.ADsSetLastError(Interop.Errors.ERROR_SUCCESS, null, null);
             }
 
-            private unsafe int GetLastError(ref int errorCode)
+            private int GetLastError(ref int errorCode)
             {
-                char c1 = '\0', c2 = '\0';
+                Span<char> errorBuffer = Span<char>.Empty;
+                Span<char> nameBuffer = Span<char>.Empty;
                 errorCode = Interop.Errors.ERROR_SUCCESS;
-                return Interop.Activeds.ADsGetLastError(out errorCode, &c1, 0, &c2, 0);
+                return Interop.Activeds.ADsGetLastError(out errorCode, errorBuffer, errorBuffer.Length, nameBuffer, nameBuffer.Length);
             }
         }
     }