todo(cas): remove unindexed regex search for existing users

[himanshu2006]

This PR resolves a lingering TODO in the CAS login flow by replacing an unindexed regex query with a direct, indexed lookup.

Previously, the fallback username check used new RegExp('^' + username + '$', 'i'). Because this utilized the case-insensitive i flag without a supporting collation index, it forced MongoDB to perform a collection scan. By replacing this with a direct { username } query, we ensure the database utilizes the standard index, preventing potential CPU spikes during CAS authentication on large instances.

[himanshu2006]

i will upload a pr soon