🧹 remove unused public function generate_op_id

[Cylae]

🎯 What: Removed the unused public function generate_op_id from server_manager/src/core/journal.rs and moved operation ID generation helper logic locally into server_manager/tests/contract_journal.rs.
💡 Why: generate_op_id was exported as a public API function in journal.rs but was never called outside of test files. Removing dead code reduces public API surface area and improves code health and maintainability.
✅ Verification: Verified with cargo fmt, cargo clippy --all-targets --all-features -- -D warnings, and full test execution (cargo test).
✨ Result: Cleaned up public module exports in journal.rs without affecting runtime functionality or breaking test coverage.

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PR created automatically by Jules for task 1337842746933228318 started by @Cylae

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