Return 'Medium' instead of 'Unavailable' if risk_band is not available

What does this MR do and why?

Resolves https://gitlab.com/gitlab-org/modelops/anti-abuse/team-tasks/-/issues/446.

In the exceptional scenario¹ where Arkose Verify API returns a JSON response without a session_risk.risk_band value, we want to treat the new user as medium-risk (required to verify their identity using email and phone number) instead of low-risk (required to verify their identity using email only).

This is achieved in this MR by falling back to returning 'Medium' instead of 'Unavailable' when calling risk_band on an Arkose::VerifyResponse object instantiated using a JSON response without session_risk.risk_band value.

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¹ This exact scenario happened 47k+ times in February, 29k+ times in January, twice in June then just stopped happening. Root cause is unknown at the moment and is actively being investigated in collaboration with Arkose team.

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