refactor(mcp): delegate load_user_with_relationships to SecurityManager.find_user_with_relationships

Fixes a gap identified in code review: the standalone load_user_with_relationships()
in auth.py duplicated SecurityManager.find_user() logic but dropped two FAB behaviors:
- auth_username_ci (case-insensitive username lookup)
- MultipleResultsFound guard (username uniqueness not guaranteed at DB level in all FAB versions)
It also hard-coded User/Group models instead of sm.user_model.

Changes:
- Add SupersetSecurityManager.find_user_with_relationships() to security/manager.py,
  mirroring FAB's find_user() (auth_username_ci, MultipleResultsFound handling,
  self.user_model) and adding eager loading of roles and group.roles via joinedload
- Simplify load_user_with_relationships() in auth.py to a thin delegate to the
  new method, removing the duplicated query logic and raw Group/User imports
- Add regression test asserting find_user_with_relationships() exists on the SM

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>