fix: async subagent background=True drops user interrupts to queue (#46864)

[argus-metis]

Summary

Fixes #46864 — delegate_task(background=True) silently blocked user messages by dropping them to queue mode instead of allowing interrupts.

Root Cause

Three interacting issues:

1. run_agent.py — _dispatch_delegate_task() didn't pass the background parameter through to delegate_task(). The downstream function has full async support (removes child from _active_children, dispatches via async_delegation) but never received the flag.

2. tools/delegate_tool.py — _build_child_agent() unconditionally registered ALL children in parent_agent._active_children for interrupt propagation. For background children, this created a race window where the child appeared briefly before the async path removed it — causing _agent_has_active_subagents() to see a non-empty list and demote user interrupts to queue.

3. gateway/run.py — _agent_has_active_subagents() had no way to distinguish background children (async, parent's tool loop has returned) from sync children (parent blocked mid-tool-loop). Both looked identical: non-empty _active_children.

Changes

run_agent.py (+1 line)

Pass background from function args through _dispatch_delegate_task() to delegate_task().

tools/delegate_tool.py (+41/-8)

• Moved child registration for interrupt propagation out of _build_child_agent() and into the sync-only call sites (delegate_task() single and batch branches).
• Background subagents are never registered — lifecycle is managed by the async-delegation registry.
• Eliminates the race window where background children briefly appeared before the async path detached them.

gateway/run.py (+15 lines)

• Added _executing_tools check: when the parent's tool loop has returned and _executing_tools is False, any remaining _active_children must be background subagents. The gateway does NOT demote user interrupts to queue.
• Safe-by-default: getattr(..., True) so attribute-missing stubs preserve the existing interrupt path.

Tests (+17 lines)

• test_returns_false_when_parent_not_executing_tools: Background children with _executing_tools=False must NOT trigger demotion.
• test_returns_false_when_executing_tools_attr_missing: Missing attribute defaults to True (backward compat).

Background

Builds on the async background delegation infrastructure from #40946 (feat(delegation): async background subagents via delegate_task(background=true)).

Checklist

• fix: commit follows conventional commit format
• Change is minimal — only touches the broken call path
• No speculative infrastructure or unrelated refactoring
• Root cause fully documented
• Tests added for new behaviour
• Existing tests pass
• No new dependencies, config keys, or tools

[teknium1]

Thanks for tracing the original background-delegation failure path. This is an automated hermes-sweeper review; current main already provides the requested non-blocking behavior.

• feat(delegation): background fan-out — parallel subagents, one consolidated return #49734 (ea8a8b4af8612b655a5bbfc74eba21e1e806758d, shipped in v2026.7.1) makes top-level model delegation unconditionally backgrounded in run_agent.py:5706-5725.
• tools/delegate_tool.py:2850-2863 detaches background children from the parent's _active_children tracking before async dispatch, so they cannot trigger synchronous-subagent interrupt demotion.
• tests/tools/test_async_delegation.py:457-486 covers the live dispatch invariant for top-level and nested delegation.
• The linked issue Async subagents queue user messages instead of allowing parallel conversation #46864 was already closed with this implementation cited.