fix(python): cover issue 1264 security gaps

[chaliy]

Summary

• add the missing Python security parity coverage from issue test(python): add missing security tests — VFS limits, arithmetic, injection, encoding #1264, including direct Bash.read_file() / write_file() / ls() / glob() cases
• fix the default sandbox shell vars so $PPID resolves to a virtual parent PID and add a regression test
• keep latest-main local pre-PR checks moving by comparing RealFs fallback paths against the canonical root and gating http_client-only hook tests

Testing

• /private/tmp/bashkit-1264-venv/bin/pytest crates/bashkit-python/tests/test_security.py -q
• /private/tmp/bashkit-1264-venv/bin/ruff check crates/bashkit-python/tests/test_security.py
• /private/tmp/bashkit-1264-venv/bin/ruff format --check crates/bashkit-python/tests/test_security.py
• cargo test -p bashkit --features http_client default_ppid_is_sandboxed -- --nocapture
• cargo test -p bashkit resolve_fallback_validates_containment -- --nocapture
• cargo test -p bashkit resolve_fallback_returns_normalized_path -- --nocapture
• just pre-pr (locally blocked on bash_comparison_tests; reproduced on clean origin/main in a detached worktree on this macOS machine)

Closes #1264.