refactor: thread app_config through lead and subagent task path (#2666)

* refactor: thread app config through lead prompt

* fix: honor explicit app config across runtime paths

* style: format subagent executor tests

* fix: thread resolved app config and guard subagents-only fallback

Address two PR review findings:

1. _create_summarization_middleware passed the original (possibly None)
   app_config into create_chat_model, forcing the model factory back to
   ambient get_app_config() and risking config drift between the
   middleware's resolved view and the model's view. Pass the resolved
   AppConfig instance through end-to-end.

2. get_available_subagent_names accepted Any-typed config and forwarded
   it to is_host_bash_allowed, which reads ``.sandbox``. A
   SubagentsAppConfig (also accepted upstream as a sum-type input) has
   no ``.sandbox`` attribute and would be silently treated as "no
   sandbox configured", incorrectly disabling the bash subagent. Guard
   on hasattr and fall back to ambient lookup otherwise.

Adds regression tests for both paths.

* chore: simplify hasattr guard and tighten regression tests

- Collapse if/else into ternary in get_available_subagent_names; hasattr(None, ...) is False so the explicit None check was redundant.
- Drop comments that narrate the change rather than explain non-obvious WHY (test names already convey intent).
- Replace stringly-typed sentinel "no-arg" in regression test with direct args tuple comparison.

---------

Co-authored-by: greatmengqi <chenmengqi.0376@bytedance.com>

backend/tests/test_invoke_acp_agent_tool.py

@@ -697,3 +697,33 @@ def test_get_available_tools_includes_invoke_acp_agent_when_agents_configured(mo
     assert "invoke_acp_agent" in [tool.name for tool in tools]
 
     load_acp_config_from_dict({})
+
+
+def test_get_available_tools_uses_explicit_app_config_for_acp_agents(monkeypatch):
+    explicit_agents = {"codex": ACPAgentConfig(command="codex-acp", description="Codex CLI")}
+    explicit_config = SimpleNamespace(
+        tools=[],
+        models=[],
+        tool_search=SimpleNamespace(enabled=False),
+        skill_evolution=SimpleNamespace(enabled=False),
+        get_model_config=lambda name: None,
+        acp_agents=explicit_agents,
+    )
+    sentinel_tool = SimpleNamespace(name="invoke_acp_agent")
+    captured: dict[str, object] = {}
+
+    def fail_get_acp_agents():
+        raise AssertionError("ambient get_acp_agents() must not be used when app_config is explicit")
+
+    def fake_build_invoke_acp_agent_tool(agents):
+        captured["agents"] = agents
+        return sentinel_tool
+
+    monkeypatch.setattr("deerflow.tools.tools.is_host_bash_allowed", lambda config=None: True)
+    monkeypatch.setattr("deerflow.config.acp_config.get_acp_agents", fail_get_acp_agents)
+    monkeypatch.setattr("deerflow.tools.builtins.invoke_acp_agent_tool.build_invoke_acp_agent_tool", fake_build_invoke_acp_agent_tool)
+
+    tools = get_available_tools(include_mcp=False, subagent_enabled=False, app_config=explicit_config)
+
+    assert captured["agents"] is explicit_agents
+    assert "invoke_acp_agent" in [tool.name for tool in tools]