R1:920ab13cfb86

Correct a possible fatal in the non-CSRF Duo MFA workflow

Summary:
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.

Test Plan: Hit Duo auth on a non-CSRF workflow (e.g., edit an MFA provider…