chore(auth-server): minor code cleanup and fixes

[sreecharan-desu]

This PR introduces a few targeted cleanups and improvements to the fxa-auth-server package:

• Fix Typo: Corrected 'seperately' to 'separately' in the OAuth API test suite.
• Unhardcode Session Token Expiry: Updated the SessionToken model to pull its expiry from the auth-server configuration instead of using a hardcoded static value.
• Added Regression Tests: Included unit tests for the session token expiry filtering logic to ensure correctness.

These changes address the maintainer's feedback regarding targeted PRs and unhardcoding configuration values, while maintaining existing functionality.

[github-actions]

This pull request issue has been marked as stale due to inactivity. It will be closed in a week if it continues to be stale.

[dschom]

Thanks for these contributions. This PR has grown stale, but you have some nice clean up here. Please see comments and feel free to refile more targeted PRs. We will try to address them in a timely manner this time around.

[sreecharan-desu]

Thanks for the feedback, @dschom! I've updated the PR to address your requests:

• Reverted Sass Improvements: Removed the changes to sass-compile-files.ts as they are no longer applicable.
• Fixed Typo: Corrected 'seperately' to 'separately' in the contemporary test file location without any formatting noise.
• Unhardcoded Session Token Expiry: The expiry is now dynamically set from the auth-server configuration instead of being a static member. I've also added unit tests in fxa-shared to verify the logic.
• Consolidated Effort: I've integrated the typo fix directly here and closed the redundant PR (fix(auth-server): correct typo 'seperately' to 'separately' in tests #20399) to keep the cleanup focused in this thread.

I hope this targeted version is much easier to review. Let me know if you need any further adjustments!