Optimize blacklist cache reload locking

[vansour]

Summary

• return shared Arc<Vec<String>> instances from the blacklist cache so readers reuse cached entries without cloning
• read the cache under a shared lock and only upgrade to a write lock when the file has changed

Testing

• cargo fmt
• cargo check
• cargo test
• cargo clippy --all-targets --all-features

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https://chatgpt.com/codex/tasks/task_e_69022389953483238b0d9b51eb82be7d

[Copilot]

Pull Request Overview

This PR optimizes the blacklist loading mechanism by addressing a race condition and reducing unnecessary cloning. The changes wrap the rules vector in an Arc to enable cheap sharing and implement a double-check locking pattern to avoid holding a write lock while checking if a reload is needed.

• Changed BlacklistCache.rules from Vec<String> to Arc<Vec<String>> for efficient sharing
• Implemented double-check locking pattern to check file modification before acquiring write lock
• Updated all usages to work with the new Arc<Vec<String>> type

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