[UI/Core Split] Refactor RconPermissionToString in Utilities

[benfleuty]

The RconPermissionToString method in Utilities.cs provides a specific conversion from an RconPermission enum to its string representation. This is a domain-specific conversion related to RCON functionality within the core.

This conversion logic should reside closer to the RconPermission enum itself or within a service related to RCON handling in the core.

Action:

1. Create an extension method for the enum in a core utility file.
2. Move the conversion logic from RconPermissionToString to the extension method in the core.
3. Update any code that uses RconPermissionToString to use the new conversion method.
4. Remove RconPermissionToString from the Utilities class.
5. Add or update tests for the RCON permission string conversion.