Refactoring: Enhance Maintainability Without Functional Changes

[PreetPatel45]

Refactoring: Enhance Maintainability Without Functional Changes

✅ Validation

✔ All existing tests pass (unit/integration)
✔ Build succeeds (mvn clean install)
✔ Manual verification of HBCK2 commands → No behavioral changes

🔧 Improvements

1. Reduced code duplication
   • New BaseMaintenanceTool abstract class for shared logic between RegionMerger and MissingRegionDirsTool.
2. Simplified complex logic
   • Extracted HBCK2CommandUsage (reduced HBCK2 by 300+ LOC).
   • Decomposed doCommandLine (cyclomatic complexity ↓ from 17).
3. Clearer threading logic
   • Introduced explaining variables (availableProcessors, threadPoolSize).

📌 Why Merge This?

• Zero-risk refactoring → Pure structural changes, no side effects.
• Easier to debug/extend → Better encapsulation, less duplication.
• Follows SOLID principles → More modular design.

💡 Review focus: Verify thread pool changes and new class structure.