🔧 refactor: Add and use PrincipalType Enum

- Replaced string literals for principal types ('user', 'group', 'public') with the new PrincipalType enum across various models, services, and tests for improved type safety and consistency.
- Updated permission handling in multiple files to utilize the PrincipalType enum, enhancing maintainability and reducing potential errors.
- Ensured all relevant tests reflect these changes to maintain coverage and functionality.

api/server/services/Files/processFiles.test.js

@@ -17,6 +17,11 @@ jest.mock('~/config', () => ({
 jest.mock('librechat-data-provider', () => ({
   isUUID: { parse: jest.fn() },
   megabyte: 1024 * 1024,
+  PrincipalType: {
+    USER: 'user',
+    GROUP: 'group',
+    PUBLIC: 'public',
+  },
   FileContext: { message_attachment: 'message_attachment' },
   FileSources: { local: 'local' },
   EModelEndpoint: { assistants: 'assistants' },