Fix empty LdapException.ServerErrorMessage (#115192)

Author: vladitekh

src/libraries/System.DirectoryServices.Protocols/src/System/DirectoryServices/Protocols/ldap/LdapSessionOptions.Linux.cs

@@ -86,6 +86,9 @@ public void StartNewTlsSessionContext()
             SetIntValueHelper(LdapOption.LDAP_OPT_X_TLS_NEWCTX, 0);
         }
 
+        // In practice, this apparently rarely if ever contains useful text
+        internal string ServerErrorMessage => GetStringValueHelper(LdapOption.LDAP_OPT_ERROR_STRING, true);
+
         private bool GetBoolValueHelper(LdapOption option)
         {
             if (_connection._disposed) throw new ObjectDisposedException(GetType().Name);

src/libraries/System.DirectoryServices.Protocols/src/System/DirectoryServices/Protocols/ldap/LdapSessionOptions.Windows.cs

@@ -51,5 +51,8 @@ public ReferralChasingOptions ReferralChasing
                 SetIntValueHelper(LdapOption.LDAP_OPT_REFERRALS, (int)value);
             }
         }
+
+        // In practice, this apparently rarely if ever contains useful text
+        internal string ServerErrorMessage => GetStringValueHelper(LdapOption.LDAP_OPT_SERVER_ERROR, true);
     }
 }

src/libraries/System.DirectoryServices.Protocols/src/System/DirectoryServices/Protocols/ldap/LdapSessionOptions.cs

@@ -522,9 +522,6 @@ public VerifyServerCertificateCallback VerifyServerCertificate
             }
         }
 
-        // In practice, this apparently rarely if ever contains useful text
-        internal string ServerErrorMessage => GetStringValueHelper(LdapOption.LDAP_OPT_SERVER_ERROR, true);
-
         internal DereferenceAlias DerefAlias
         {
             get => (DereferenceAlias)GetIntValueHelper(LdapOption.LDAP_OPT_DEREF);