This fix restores and extends the username/email search functionality across workspace pages that was originally added in PR #14002. The issue was that:
1. The backend search functions for Models and Knowledge only searched `User.name` and `User.email`, but not `User.username`
2. The Functions admin page lacked user search entirely
Changes made:
Added User.username to backend search conditions for Models and Knowledge pages
Added complete user search (name, email, username) to the Functions admin page client-side filter
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