Fix user creation failure handling in OAuth paths (#29207)

Co-authored-by: Matt Jankowski <matt@jankowski.online>

spec/support/omniauth_mocks.rb

@@ -0,0 +1,7 @@
+# frozen_string_literal: true
+
+OmniAuth.config.test_mode = true
+
+def mock_omniauth(provider, data)
+  OmniAuth.config.mock_auth[provider] = OmniAuth::AuthHash.new(data)
+end