Ref T13259. If we miss the separate CSRF step in Duo and proceed directly to prompting, we may fail to build a response which turns into a real control and fatal on null->setLabel().

Instead, let MFA providers customize their "bare prompt dialog" response, then make Duo use the same "you have an outstanding request" response for the CSRF and no-CSRF workflows.

Hit Duo auth on a non-CSRF workflow (e.g., edit an MFA provider with Duo enabled). Previously: setLabel() fatal. After patch: smooth sailing.