🔧 refactor: Integrate PrincipalModel Enum for Principal Handling

- Replaced string literals for principal models ('User', 'Group') with the new PrincipalModel enum across various models, services, and tests to enhance type safety and consistency.
- Updated permission handling in multiple files to utilize the PrincipalModel enum, improving maintainability and reducing potential errors.
- Ensured all relevant tests reflect these changes to maintain coverage and functionality.

packages/data-provider/src/accessPermissions.ts

@@ -19,6 +19,14 @@ export enum PrincipalType {
   PUBLIC = 'public',
 }
 
+/**
+ * Principal model types for MongoDB references
+ */
+export enum PrincipalModel {
+  USER = 'User',
+  GROUP = 'Group',
+}
+
 /**
  * Source of the principal (local LibreChat or external Entra ID)
  */