Improve error messages for authentication setup and credentials

[avanderbergh]

Description

relates to #6097

Fix two incorrect error messages in the core auth package:

• Correct the Workload Identity Federation setup error so it no longer reports no auth
• Correct the corrupted credentials unmarshalling error message

Also add regression tests for both error-message paths and update the core changelog/version for the next core patch release.

Note: this PR updates the core changelog and core VERSION file, not a service changelog or service VERSION file.

Checklist

• Issue was linked above
• No generated code was adjusted manually (check comments in file header)
• Changelogs
  • Changelog in the root directory was adjusted (see here)
  • Changelog(s) of the service(s) were adjusted (see e.g. here)
• VERSION file(s) of the service(s) were adjusted
• Code format was applied: make fmt
• Examples were added / adjusted (see examples/ directory)
• Unit tests got implemented or updated
• Unit tests are passing: make test (will be checked by CI)
• No linter issues: make lint (will be checked by CI)

Core module note:core/CHANGELOG.md and [x] core/VERSION were adjusted

[marceljk]

Thanks for your contribution! Looks good to me, only one nitpick regarding the changelog