🧹 Remove unused validate_domain public function

[Cylae]

🎯 What: Removed the unused public function validate_domain from server_manager/src/core/validate.rs and deleted its associated test case test_validate_domain in server_manager/tests/contract_input_validation.rs.

💡 Why: As server_manager is a binary crate, public functions that are not used anywhere within the crate constitute dead code. Removing unused code improves codebase maintainability and reduces clutter.

✅ Verification:

• Ran cargo fmt to ensure formatting compliance.
• Ran cargo clippy --manifest-path server_manager/Cargo.toml --all-targets --all-features -- -D warnings to confirm zero lint warnings.
• Ran cargo test --manifest-path server_manager/Cargo.toml --all-targets --all-features to verify that all remaining unit, contract, integration tests, and benchmarks pass.

✨ Result: Cleaned up dead code and improved overall codebase health.

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PR created automatically by Jules for task 2958005753390998787 started by @Cylae

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