fix(sandbox): finish dependency cleanup on cancellation

[hsusul]

Summary

Dependencies.aclose() stopped reverse-order cleanup as soon as an owned dependency raised CancelledError. That left earlier owned resources open and cached a cancelled close task, so subsequent close attempts could not complete the skipped cleanup.

This change records the first child cancellation, finishes exactly-once cleanup for the remaining owned dependencies, clears the container's internal state, and then propagates cancellation. Normal cleanup and non-cancellation best-effort behavior are unchanged.

Test plan

• Added a regression with two owned dependencies proving that a later dependency's cancellation does not skip the earlier resource, cancellation still propagates, and a repeated aclose() does not close either resource twice.
• uv run pytest tests/sandbox/test_dependencies.py::test_dependencies_aclose_finishes_owned_cleanup_before_propagating_cancellation -q — 1 passed; repeated 10 times with the same result.
• uv run pytest tests/sandbox/test_dependencies.py -q — 20 passed.
• make typecheck — mypy passed for 307 source files; Pyright reported 0 errors.
• .agents/skills/code-change-verification/scripts/run.sh — format and lint passed; the broad suite completed with 9085 passed, 28 skipped, and 1 pre-existing failure. tests/sandbox/test_run_cwd.py::test_python_skill_uses_absolute_root_from_nested_workdir fails with the same missing output file on clean upstream 23346799 and is unrelated to this two-file cleanup change.
• git diff --check — passed.

No public API, serialization format, or documentation changes.

Issue number

None filed; reproduced deterministically with local owned-resource fakes.

Checks

• I've added new tests, if relevant
• I've run .agents/skills/code-change-verification/scripts/run.sh
• I've confirmed all verification steps pass
• If using Codex, I've run /review before submitting this PR