🧪 test: add unit test for root privilege check

[Cylae]

🎯 What: Added unit test coverage for is_root and check_root in server_manager/src/core/system.rs.
📊 Coverage: Tests verify is_root() matching nix::unistd::Uid::effective().is_root() and check_root() returning Ok(()) or Err("This application must be run as root.") depending on effective process UID.
✨ Result: Improved test coverage and deterministic behavior verification for root permission checks.

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PR created automatically by Jules for task 1541694811706861378 started by @Cylae

[google-labs-jules]

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