Backport of 'Fail closed when OpenBao login returns a non-JSON body'

What does this MR do and why?

Backport of !263531 (merged) to 19.5.

When an OpenBao call in the Secrets Manager capabilities_self flow gets a response that is not JSON (for example, an empty 307 from an OpenBao standby), the client raised NoMethodError. This error is not rescued by EffectiveCapabilitiesService, so the sidebar check turned the whole page into a 500. Users hit it again on every page that renders the sidebar for that group or project.

The fix makes the client raise ApiError or AuthenticationError instead. The service already rescues these and fails closed, so the menu item is hidden and the page renders.

This is a clean cherry-pick of the original commit, with no conflicts.

References

• Original MR: !263531 (merged)
• Bug issue: #634776 (closed)

MR acceptance checklist

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• This MR is backporting a bug fix, documentation update, or spec fix, previously merged in the default branch.
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