Harden reverseshell.py and add unit test coverage

[adjusic]

This PR improves the security of reverseshell.py and adds proper test coverage.

Fixes:

• Replaces os.system() with subprocess.run() for safer command execution.
• Validates all user inputs (e.g., shell type, yes/no prompts).
• Removes unsafe or unnecessary system calls.
• Adds graceful fallback handling.

Testing:

• Added tests in tests/test_reverseshell.py
• All tests pass via pytest
• Manual tests confirm shell selection and payload generation work as expected

Result:

• File now passes static analysis in security_scan.py
• Hardened against input injection and misuse