fix: add SAML challenge to reauth (#819)

* fix: add SAML challenge to reauth

* add enable_reauth_refresh flag

* address comments

* fix unit test

* address comments

* update

* update

* update

* update

* 🦉 Updates from OwlBot

See https://github.com/googleapis/repo-automation-bots/blob/main/packages/owl-bot/README.md

Co-authored-by: Owl Bot <gcf-owl-bot[bot]@users.noreply.github.com>
Co-authored-by: Tres Seaver <tseaver@palladion.com>
diff --git a/tests/oauth2/test_challenges.py b/tests/oauth2/test_challenges.py
index 019b908..412895a 100644
--- a/tests/oauth2/test_challenges.py
+++ b/tests/oauth2/test_challenges.py
@@ -130,3 +130,11 @@
         assert challenges.PasswordChallenge().obtain_challenge_input({}) == {
             "credential": " "
         }
+
+
+def test_saml_challenge():
+    challenge = challenges.SamlChallenge()
+    assert challenge.is_locally_eligible
+    assert challenge.name == "SAML"
+    with pytest.raises(exceptions.ReauthSamlChallengeFailError):
+        challenge.obtain_challenge_input(None)