The validate_path() function in file_tools.py used str.startswith() to
verify that a resolved path stays within the sandbox root. This allowed
sibling directories whose names share a string prefix with the sandbox
(e.g. /tmp/sandbox-escape/ bypassing /tmp/sandbox/) to escape the
containment check.
Replace the string prefix check with Path.relative_to(root), which
performs a proper OS-level path prefix comparison using path separators.
Add a Behave regression scenario tagged @tdd_issue @tdd_issue_7558 that
exercises the prefix-collision bypass to prevent regressions.
ISSUES CLOSED: #7558
HAL9000