fix: add username search support to workspace and admin pages (#20780)

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

backend/open_webui/models/knowledge.py

@@ -229,6 +229,9 @@ class KnowledgeTable:
                             or_(
                                 Knowledge.name.ilike(f"%{query_key}%"),
                                 Knowledge.description.ilike(f"%{query_key}%"),
+                                User.name.ilike(f"%{query_key}%"),
+                                User.email.ilike(f"%{query_key}%"),
+                                User.username.ilike(f"%{query_key}%"),
                             )
                         )