Refactor thread count calculation in main function to use 80% of avai…

…lable CPU cores, ensuring a more efficient default setting.

Enhance error handling and improve test coverage in run-tests.sh

- Updated exit code handling in run-tests.sh to ensure consistent error reporting across tests, improving clarity in test results.
- Added new tests for edge cases related to deletion operations, including handling of protected paths and concurrent deletions, enhancing overall test coverage.
- Improved output messages during test execution for better debugging and validation of deletion functionality.