Enum that represents the challenge option for challenge-based
(CHECKBOX, INVISIBLE) testing keys.
Values:
TESTING_CHALLENGE_UNSPECIFIED (0):
Perform the normal risk analysis and return
either nocaptcha or a challenge depending on
risk and trust factors.
NOCAPTCHA (1):
Challenge requests for this key always return
a nocaptcha, which does not require a solution.
UNSOLVABLE_CHALLENGE (2):
Challenge requests for this key always return
an unsolvable challenge.
