Step-up Auth: StepUpAuthenticationFlow handle empty step-up auth session

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What does this MR do and why?

Step-up Auth: StepUpAuthenticationFlow handle empty step-up auth session

StepUpAuthenticationFlow was not able to handle an empty session, which could lead to errors when trying to access session data. This commit adds checks to ensure that the session data is present before attempting to access it.

This commit also refactors the tests for better readability and maintainability.

Changelog: other

🛠️ with ❤️ at Siemens

References

• #474650

Screenshots or screen recordings

Only backend refacotring changes. No changes in behavior.

How to set up and validate locally

1. Run the test

bundle exec rspec spec/lib/gitlab/auth/oidc/step_up_authentication_flow_spec.rb

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

MR Checklist (@gerardo-navarro)

• Changelog entry added, if necessary
• Documentation created/updated via this MR
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the style guides
• Conforms to the javascript style guides
• Conforms to the database guides
• Conforms to the merge request performance guidelines

Related to #474650