Improve SGU logon resilience and client UX

src/SGU.AuthBroker/Services/NtlmCredentialValidator.cs

@@ -1,5 +1,4 @@
 using System.Net;
-using System.Text;
 using SGU.AuthBroker.Core.Authentication;
 using SGU.AuthBroker.Core.Identity;
 using SGU.AuthBroker.Core.Profiles;
@@ -202,20 +201,9 @@ public sealed class NtlmCredentialValidator(BrokerOptions options) : INtlmCreden
             buffer.Write(chunk, 0, read);
         }
 
-        string? charset = content.Headers.ContentType?.CharSet?.Trim('"', '\'');
-        Encoding encoding;
-        try
-        {
-            encoding = string.IsNullOrWhiteSpace(charset)
-                ? Encoding.UTF8
-                : Encoding.GetEncoding(charset);
-        }
-        catch (ArgumentException)
-        {
-            encoding = Encoding.UTF8;
-        }
-
-        return encoding.GetString(buffer.GetBuffer(), 0, checked((int)buffer.Length));
+        return SguHtmlDecoder.Decode(
+            buffer.GetBuffer().AsSpan(0, checked((int)buffer.Length)),
+            content.Headers.ContentType?.CharSet);
     }
 
     private static bool IsAllowedHttpsUri(Uri uri, HashSet<string> allowedHosts) =>