Fix an issue where Duo validation could incorrectly apply to other factor types

See <https://discourse.phabricator-community.org/t/configuring-mfa-provider-totp-fails-for-missing-duo-only-options/2355>.

Test Plan: Created a TOTP provider; created a Duo provider (with missing and supplied values).

src/applications/auth/xaction/PhabricatorAuthFactorProviderDuoCredentialTransaction.php

···
     $actor = $this->getActor();
     $errors = array();
 
+    if (!$this->isDuoProvider($object)) {
+      return $errors;
+    }
+
     $old_value = $this->generateOldValue($object);
     if ($this->isEmptyTextTransaction($old_value, $xactions)) {
       $errors[] = $this->newRequiredError(

src/applications/auth/xaction/PhabricatorAuthFactorProviderDuoHostnameTransaction.php

···
   public function validateTransactions($object, array $xactions) {
     $errors = array();
 
+    if (!$this->isDuoProvider($object)) {
+      return $errors;
+    }
+
     $old_value = $this->generateOldValue($object);
     if ($this->isEmptyTextTransaction($old_value, $xactions)) {
       $errors[] = $this->newRequiredError(

src/applications/auth/xaction/PhabricatorAuthFactorProviderTransactionType.php

···
 <?php
 
 abstract class PhabricatorAuthFactorProviderTransactionType
-  extends PhabricatorModularTransactionType {}
+  extends PhabricatorModularTransactionType {
+
+  final protected function isDuoProvider(
+    PhabricatorAuthFactorProvider $provider) {
+    $duo_key = id(new PhabricatorDuoAuthFactor())->getFactorKey();
+    return ($provider->getProviderFactorKey() === $duo_key);
+  }
+
+}