Attempt other auth flow if sharedkey is invalid

[equinor-ruaj]

Ref. case https://equinor.slack.com/archives/CT986DYF3/p1769596454265799 where user upload failed due to sharedkey that was invalid. (Signed by previous version of Sumo-core and could not be validated by the new due to new set of keys)
Related issue: equinor/fmu-sumo-sim2sumo#192

We should:
If sharedkey validation fails: try to authorize with some other mechanism (fallback)
If that also fails: fail as normal

To decide:
Are there some cases/error codes where we do not need to try another method?
Or should we just always try a fallback if sharedkey fails? I.e. treat the sharedkey as a "bonus"

[equinor-ruaj]

11.02.2026:
Did a mob programming session on the suggested PR.
We came to the conclusion that we do not want to implement this fix now.
The issue it solves is very rare and will most likely only happen when we rotate signing keys or something similar (as was the case that triggered this task)

Putting this on ice for now. (We´ll keep the PR branch for a while in case)