fix(soul): cancel nested task on outer cancellation

[koriyoshi2041]

Summary

• include the initial asyncio.wait() in run_soul lifecycle cleanup
• cancel and await nested soul/cancel-event tasks when the outer coroutine is cancelled
• add a regression for cancellation while the nested soul task is still running

Fixes #2615.

Validation

• uv run pytest tests/core tests/background -q (919 passed)
• uv run pyright src/kimi_cli/soul/__init__.py tests/core/test_run_soul_cancellation.py
• uv run ruff check src/kimi_cli/soul/__init__.py tests/core/test_run_soul_cancellation.py
• uv run ruff format --check src/kimi_cli/soul/__init__.py tests/core/test_run_soul_cancellation.py
• git diff --check

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[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no bugs or issues to report.