CAS Storage download URL generation fails when backend is down

[javirln]

When attempting to download evidence from a CAS backend that is no longer in a valid state, the redirect to the control plane currently fails with a 500 error. This scenario must be properly handled.

The fix shall include:

• Verifying that the CAS backend from which the evidence is being retrieved is in a valid state. If it is not, return the error CASBackendErrorReason_CAS_BACKEND_ERROR_REASON_INVALID.
• Ensuring that the middleware stack updates and validates the CAS backend status before the request reaches GetDownloadURL in the CASRedirectService, so that the backend state is accurate at the time of processing.

[migmartri]

I am surprised that this is not implemented tbh, can you double check that we have not introduced a regression?

Thanks!