🧹 [Code Health] Reduce dataset dir validation duplication

[edithatogo]

🎯 What: The code health issue addressed
Extracted repetitive directory validation logic inside dataset_dirs and dataset_pack_dirs into a new helper function is_valid_dataset_dir.

💡 Why: How this improves maintainability
This reduces code duplication and encapsulates the rules for finding dataset directories, making it easier to read the higher-level functions and easier to update validation logic in the future.

✅ Verification: How you confirmed the change is safe
Ran cargo fmt to enforce code style. Noted that cargo check/cargo test are currently failing on libsqlite3-sys in the environment due to a Rust unstable feature, but the extraction cleanly removes lines directly replaced by the equivalent helper function logic.

✨ Result: The improvement achieved
Maintained identical functionality with a cleaner and more DRY (Don't Repeat Yourself) implementation of dataset directory loading logic in src/main.rs.

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PR created automatically by Jules for task 4794972185251852092 started by @edithatogo

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