fix(delegation): forward background flag to delegate task

[tmcginn54]

Summary

• forward the delegate_task background flag through AIAgent._dispatch_delegate_task
• add a regression test covering async background delegation dispatch

Test Plan

• uv run --with pytest python -m pytest tests/run_agent/test_run_agent.py::test_dispatch_delegate_task_forwards_background_flag -q -o 'addopts='
• uv run python -m py_compile run_agent.py tests/run_agent/test_run_agent.py
• uv run --with pytest python -m pytest tests/tools/test_async_delegation.py -q -o 'addopts='

Notes

Before this fix, model/tool calls that included background=true were silently dispatched synchronously because the flag was dropped at the AIAgent forwarding layer.

[tmcginn54]

Closing as duplicate/already fixed upstream by #46968. I missed that origin/main had moved before opening this; #46968 contains the same runtime fix for forwarding background through AIAgent._dispatch_delegate_task.