security: fix path traversal vulnerability in sidecar file endpoints

- Replace vulnerable str().startswith() with Path.is_relative_to()
  for proper path containment validation (fixes prefix collision attacks)
- Add missing shutil import for directory deletion
- Add protection against deleting the working directory itself
- Add unit tests demonstrating the vulnerability and fix

Fixes: wipash/K8sInterpreter#5, wipash/K8sInterpreter#7