test: avoid leaking coroutine in timeout test

[JoshSalomon]

Summary

• exercise the real asyncio timeout and cancellation path in the fetcher timeout test
• eliminate the unawaited AsyncMock coroutine warning
• preserve assertions for RefResolutionError and process termination

Testing

• uv run pytest tests/unit/ -q: 2786 passed
• uv run pytest tests/unit/skills/test_fetcher.py -q -W error: 34 passed
• uv run ruff check tests/unit/skills/test_fetcher.py