Automation: Update user retention tests

[yonasberhe23]

Summary

Fixes #rancher/qa-tasks#2066

• Move user retention tests from cypress/e2e/tests/pages/users-and-auth/index.spec.ts to their own dedicated test file to improve test organization.
• All existing user retention test cases preserved and improved
• Form validation, save functionality, and user account blocking scenarios remain intact

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Checklist

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