🧹 remove unused is_public method in PortMapping

[Cylae]

🎯 What:
Removed the unused public method is_public from PortMapping in server_manager/src/services/mod.rs and added unit test coverage for PortMapping methods.

💡 Why:
is_public was dead code not referenced anywhere in the codebase. Removing unused code improves code health, maintainability, and readability.

✅ Verification:
Ran cargo test and cargo clippy locally, confirming all 16 unit tests and integration tests pass cleanly with zero warnings.

✨ Result:
Cleaned up dead code in PortMapping and expanded test coverage.

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PR created automatically by Jules for task 9151569566518202775 started by @Cylae

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